the "odd" and "even" is a clue

8088 systems used an 8-bit data bus so they only needed one 8-bit ROM, when 286 came along with its 16-bit data bus they decided to interleave bytes on to 2 8-bit ROMs to keep using the same EPROMS

most 386 boards and many 486 boards did this as well

8-bit ROMs were simply cheaper. Two are needed due to the 16-bit bus. Every second byte is stored in the other ROM chip.

This was also common on the Amiga. They used two cheaper 16-bit ROMs for the Amiga 1200/3000/4000.

Early chipsets didn't try to support single 8bit ROM and/or shadowing.
When shadowing isn't possible and you care about performance you use two chips to fill the entire bus for increased performance rather than splitting up bus cycles into several smaller ones. With shadowing, access to ROM area goes into (usually) write protected RAM at same addresses, increasing performance further since RAM has faster (page mode) access speeds than any EPROMs or flash chips.

One of my 286 boards (PCChips M209) only has one 8bit EPROM, but it's still split internally into odd/even (first half vs. last half of the chip). How...odd is that? 😀

Also have a 386SX SBC, sadly damaged beyond repair, that uses a single 16bit EPROM (27C1024).

quicknick wrote on 2020-04-28, 19:33:

How...odd is that? 😀

well it's about half odd.

Oh, but there are 286s with single BIOS EPROM! Also most older/cheaper 386 and 486 use a single 512k EPROM, and it's still 8-bit how about that? The answer is - single ROM in 286 needs multiplexing/demultiplexing which complicates things (a bit). Two ROMs do not need it. I would argue that it was cost releated in the later boards, single larger EPROM should cost less than 2 smaller EPROMS (and count the sockets in and so on) - I think it was just legacy thing and maybe the mux/demux of a single ROM would add some complications too.