Sometimes, when I tip a keg and see that it is very low, I'll bottle the remaining beer and put it in my beer fridge or cellar it, or to just have an alternative to what remains on tap, and/or to share with friends. It also helps to get rid of the anxiety of "will I have room for the next two kegs almost due to fit in here?"
I went on a fishing trip last weekend and before heading out had a low keg of Dubbel and was bottling other beers too to share with a fishing buddy from another state, and so bottled what I figured would be two or three beers left of the dubbel, but ended up bottling eight of them. No big deal, I keep lots of empty bottles and caps on hand for just such situations.