Probably XT machines?

That are the drives i am using on my XT, or other old computer like C64. In vintage computering most of cases the floppy drive is the only way to import files into the computer.

You can try using a serial (or parallel) cable connection with utilities like Laplink but, modern computers don't seem to have serial and/or parallel ports anymore. I usually use an "in between" PC for that purpose (like a Windows 98 PC/laptop with LAN & USB connectivity). From there, you can then transfer files via serial (or parallel) cable to your XT PC.

Or use xt-ide and cf 😀

I have a floppy drive modded to be 720k only for amstrad CPC stuff.

Yes, some systems have it as the only boot device ....plus some certain synthesizers and samplers use it. 😉

I had such question just that perhaps someone might have another idea besides commodore64 and XT computers, becouse I have not used that 5.25" drive for 20 years. Not even its 1.2MB floppys.
So far the reason might be that a) 486 is the only machine which has it; b) the most files are big for a floppy transfer in my 486; c) all other machines and synthesizers have 3.5" drive.
d) for transfer the files in the machines the NFS with LAN is used; e) the boot floppys are all 3.5".

So far the only use for a floppy is as boot floppy (even on pentium) and thats too the 1.44MB becouse 1) not every machine does not boot from CD/USB/LAN ; 2) the partitioned HDDs are not compatible or transferrable between machines as HDD partitions with a overlay driver are not recognized correctly in pentiums.

So the floppy remains then the most viable bootable device on these systems and the 1.44MB 3.5" is the only one in common with other computers.

Experiments with Compact Flash (4GB and 1GB tested) drives:

just format-and-swap between machines wont work always. It is just like HDD swapping between machines as they are not transferrable between machines.
Before the CF can be used in 486SX, it has to be formatted in that system first but then use it in the CF-reader or IDE interface on pentiums (linux/dos/win9x/winxp) does not recognize it correctly. So far it leads to conclusion that once it is on the IDE bus connected, it stays on exactly the same machine connected.
On the SCSI-PCMCIA Compact Flash adapter it has errors too,
WIth SCSI-IDE adapter to Compact Flash it has given positive results, and so it can be used.

If I recall correctly, I never had 5 1/4" drives nor did I know anyone with them. My XT and 386DX-33 both came with 3,5" ones.
My best guess is that back in the days, programs would fit on a single floppy or we didn't mind swapping them a lot 😀
If I use them today, it's to make the machines as original as possible.

Recently I bought a Tandy 1000, an IBM 5160 and an XT clone and these all came with the 360k drives.

(an edit of the article above to add the experiments with Compact flash medium)

The 5.25" 1.2MB drives were more expensive back then compared to 3.5" 1.44MB drives when the 386 were for sale in shops during 1992..1993. (I recall the price difference aprox 30..45%),
while the 360kB drives were not for sale any longer in shops.
ALso those 1.2MB drives were more prone to failure than 360kB drives (so that floppy formatted on one 1.2MB drive was not readable in other 1.2MB drive).

But of course, back then not every second data file was a half of the megabyte as it is now, it it was rather 20..30kB.
The operating system was very small too. Also data formats were small. For a one floppy disk fit the entire database of the year sales of a small company.

by the way... on the small computers, there were also one-sided 5.25" drives -- 160kB 180kB capacity and when using the double-sided 360kB floppys with those drives then it was not only swapping the floppys, but also turning the side. 😁 😉 180kB written on one side and then you turn around and 180kB written on another side.

Our 286 had both sizes of floppies back in 1991. Back then cover disks for PC magazines were usually 5.25", since they were cheaper to manufacture, and they were often 360k to be compatible with most machines. Our 486SX in 1993 also came with both sizes, but when upgrading to a CD ROM I got rid of the 5.25" drive.

I have a Gotek that I will mod and install in the 286, and I have plans to get a 5.25" drive for the nostalgic feeling. The CF card and the Gotek already make any floppy obsolete, but I think it will be a lot of fun to restore and use a 5.25" drive.

I had a Charmilles wire erosion machine in a manufacturing facility that I was responsible from (first half of 2000s, the machine was from 1996 or 97). Its industrial PC was only supporting 720KB floppy drives. At that time, 1.44MB drives had a price like 5 USD a pieace, new, but 720 KB drives were already long göne. Charmilles technical support was selling them for 300+ USDs.

A couple of my 286-based Toshiba portables (1986-1987) and IIRC one of my 386 Toshiba portables (~1987) only work with 720K floppies. Conversely, I have a WYSE 286 system (Exact age unknown, but assuming about the same age because of EGA graphics) that has both a 1.2MB 5.25in drive and 1.44MB 3.5in drive...although admittedly one or both of those could have been upgraded by the previous owner (I have no evidence suggesting they were upgraded, though).

I also have an Amstrad PPC640 from 1988 with dual 720KB floppy drives, but the only reason it uses 720KB floppies so late in the game is because it was designed to be as utterly inexpensive as possible.

I also remember in 1993 getting a then-new Smith Corona electronic word processor that only accepted DOS-formatted 720KB floppies (Although it did not run DOS), but again I imagine the desire to use the cheapest possible parts played a role here. Also, the fact that it's more of a specialty product as opposed to a general-purpose computer, so they could get by with using lower-end and older hardware.

My ~1986 AT&T PC 6300 has two 360KB 5.25in floppy drives, with AFAIK no support for anything larger/newer.

I have no hands-on experience with them, but I know the 720KB (Or rather 800KB) 3.5in floppy was what several models of Macintosh used for quite a while during the 80s. In fact, IIRC the first couple models even used 400KB 3.5in floppies.

So, basically, aside from on the Mac side, 720KB 3.5in floppies was common/the norm from when the 3.5in disks were first introduced through about ~1987 or so I would estimate, while 360KB 5.25in floppies (Or 180KB if the drive was only single-sided) were common ~1979-1985 or so. Of course, none of these are firm cutoff dates as I've demonstrated above, more of a broad "most commonly used" time period based on my observations. I'm sure others with significantly more vintage computing experience than me could give more accurate timelines.