And another reason to do fanservice: it makes the audience curious. Are they really together, is it an act? Who are these people who are seemingly OK with sharing sides of themselves that most people keep private?

I think Infinite (and/or their management company) understand this principle very well. The point is to intrigue the audience. Big Bang are good at this too, but in their case they are juggling twin urges towards secrecy and confession (and not maintaining strong boundaries between fantasy and reality); Infinite, on the other hand, are mysterious because they rotate the fanservice around so it's not clear whose idea it was in the first place.