The original SideWinder 3D Pro was never USB. That's why the 3DP-Vert USB adapter was made to begin with.
The later "3D Pro Plus"/Precision Pro is where things get a bit confusing, mainly because the earlier variants were also gameport-only while lacking the analog emulation modes of the prior 3D Pro entirely. The later variants added a native USB interface internally and have a few extra pins on its gameport connector to represent that; naturally, its USB adapter cable just connects to the pins in question and lacks any active circuitry of its own.
As for the 3D Pro itself, it was the stick I did most of my late 1990s gaming with, so there's a bit of a nostalgia element involved.
I didn't have a whole lot of respect for it back then because I had lusted over the HOTAS setups I saw on store shelves and noted how many things I couldn't bind to the 3D Pro due to lack of buttons, but nowadays, I have a bit more respect for its optical sensor system (no more spiky potentiometers!) and the fact that it can emulate the TM FCS and CH Flightstick Pro to begin with, making it compatible with a lot of those early DOS-era sims.
I just wish I could actually get the bloody thing to work in digital mode on my P4 box. All those alternate timing drivers I tried didn't seem to help one bit, it's still coming up as "Not Connected".