I haven’t voted for any options because my feeling is–basically any reason is a good reason to fave an observation. While favorites are indeed used as a way to sort photos or find popular observations, I approach them as being a tool for the person who is doing the favoriting first and foremost. If it is supposed to serve some other specific community purpose, like “vote for observation of the day” or “say this is a wonderful photo and should be rated highly so other people see this photo first” then it should be named something that is less subjective than “is this observation, in some way, your ‘favorite’?”
Broadly speaking, I favorite observations because I want a collection of my favorites to be able to look through later. This means I tend to favorite for two reasons: because I find an observation particularly beautiful, or because I find it (or the discussion associated with it) interesting.
I disagree with this primarily because I think that the things people are favoriting for bookmarks is “the good stuff”. People do not bookmark stuff at random. They bookmark stuff because there’s something interesting here. It might be beautiful, or be the first of its kind, or have a good discussion, or be statistically significant, or show some interesting anomaly, or anything else. Multiple people favoriting (bookmarking!) something happens for a reason.
In fact, I think using it as a bookmark means that people are going to actually be using it for things that are genuinely interesting, rather than things like the one poll option I just noticed “the observer is your friend”. I have friends on iNat but I don’t fave their obs automatically because my faves exist to be a catalog of interesting things and not just handing out internet points to someone I like.