I used to use a stick like this one in conjunction with my XG Flash 128mbit when the save battery started dying on me. With it I would finish playing a ROM off of the XG Flash, turn the system off then immediately on (with the memory stick in while holding Start & Select to get to the memory stick's menu), and I'd be able to back up the save before it was lost. Then, when I was starting again, I'd boot up the memory stick's menu, transfer the save back to the cart and continue playing.
Eventually I just bought a better flash cart. :/
EDIT:
Apparently mine is a super Memory Stick.
Here's a photo where I still have some of the saves backed up:
XGBootLoader is the name of the save file it creates, but it works.
Sadly, my XG Flash doesn't seem to want to work anymore. Ah well. It was a shitty cart and I have a lot of fun with it.
For those who are interested, this is what mine looks like:
It's old and the writing is long worn away.