All depending on what they were - you might wish someday you had kept one of them!
As somebody who "collects" old computer crap (ok, ok - hoards might be a better term), I had a situation not too long back where I needed one of those old adapters.
I had purchased an AMD motherboard that had onboard VGA; having gone so long since I last upgraded, I didn't read the fine print saying that onboard VGA was dependent on having an AMD processor with VGA built in (APU I think they called it). I had purchased a different processor for the socket, without that feature - thinking "naw, don't need it, because I'm gonna drop my GPU in there - and switch the BIOS over to use it".
I get everything together and I boot it. I get nothing on the monitor, and beeping - POST code saying no video, and I'm like "wtf?". Some quick research and such reveals my mistake: Without the APU version of the CPU, I can't change the BIOS to use the GPU in the PCI-E slot. But I don't want to spend a boatload more money on a CPU just to get this setup. So I have a plan: Somewhere out in my junk I know I have a PCI slot VGA card (like an old S3 Virge or such) - I just have to find it.
An hour of digging around finally nets me the card - has 4 meg on it - I figure that's enough to see the BIOS, right?
I boot it - and it sorta comes up, but everything's all glitched out, nothing's really working right, then it dumps to a screen with a "not enough video memory" error, and now I'm wondering "huh?". So I go dig some more...
30 minutes of digging, I find a different PCI video card with 8 meg; surely this will work. I slot it in and boot it; finally it boots - and I am greeted with this entire super-graphical fancy GUI for a BIOS setup, complete with mouse and everything! That's how long since I last did anything PC building-wise (I was replacing a Core2Duo machine); my last BIOS was textual. It was pretty apparent why it ran out of memory; honestly, I thought it was stupid - but what'r gunna do, right?
So after some orientation and such - I find where on the BIOS it was looking for the video card (of course, defaulting to "on board" - but you would think that if it didn't find it, it would then try the PCI-E slot, but nooooo), change it out, save and shutdown, then re-install my GPU on the PCI-E slot, reboot...
...and it worked.
Had I not had that old VGA card (granted, it's not the oldest I have - I have an EISA VGA card somewhere) - I would've been hosed.
