Thanks so much!
I can't speak for everyone, but the reason I do it occasionally is because I have enough extra/event photos for one page, but not two in order to keep my page count even. On my pages above, in the first example, I had some cute cookie baking photos that went with that weekend, but were sort of separate from all of the outdoor photos I was using. I didn't want to stretch them to fit a two page spread, so I did a small 6x12 "insert" with those photos. Same with the second set, with my daughter's school dance, I had several photos I wanted to use, but I didn't want to make it its own two page spread. If the event is big enough and I have enough photos, I will make it its own two page spread... but when I have just enough for one page, I split them up to fit a 6x12 template, print them back to back and trim the excess when my book comes in. I hope that makes sense! It's one of those things that makes sense to me in my head, but is hard to type out.