No idea if this sort of question is allowed, but it can't hurt to try.
I was looking at my freshman year yearbook and I started wondering. It's a well made, 200+ page book for my 2000 person high school, documenting the clubs, people, and popular culture (senior quotes) of the time. But what other sorts of things could a historian learn from this sort of book?
Well this is weirdly specific, but I'm actually trying to track down a yearbook to find interior pictures of a theater. This theater used to be top of of the line, but didn't work financially and folded in a few years, was gutted and turned into a distillery. Later on that went out of business to, and now there are some folks looking to restore the building as a theater, but there are NO pictures of the interior.
I found a graduation announcement in the newspaper from its I operating years that said the ceremony would occur in the theater. Of course this is one of the few years the local historical society does not have.
So. I'm trying to use one for restoration of a historic building.
Oh goodness, so much!
Off the top of my head:
History of fashion. Future historians will have magazines and archived websites from stores and social media. But a yearbook will give a snapshot of what certain people in certain places were actually wearing.
Cultural history. If a historian had a few yearbooks from one place spanning a good number of years, they could be a great litmus test for cultural change. If, for instance, a high school has a 4-H club shut down and an LGBT club start up, then that probably (though I realise I'm engaging in a fair amount of stereotype here!) speaks to an overall cultural shift.
History of Education. What new classes are being offered? Which teachers are the most popular and why? Are most members of the graduating class going to college and, if so, where? Are art and music programs shutting? Are athletics programs growing?
Reactions to major events. I would imagine that lots of high school yearbooks this year will have stuff about people's reaction to the US election, for instance. While it's true that social media will be an enormous resource to future historians, something like a yearbook will provide a snapshot.
Yearbooks will also, in addition to documenting the past, be fantastic evidence for how people wanted to be perceived, which is something historians find fascinating. What if a school is involved in a scandal, evidenced by news coverage and social media posts, but the yearbook that year doesn't mention it?
So I'm not saying you need to preserve your yearbooks at all costs (goodness knows I couldn't tell you now where mine are), but as a historical resource, they will be a lot of fun.