I think different people could do the same thing for different reasons. For me it's just about catching people up, and sometimes people return the favor.
Regarding social networks, though - people aren't exactly looking at your photos because they want to. Most social networks decide what you see, so the algorithm will put vacation photos in front of you even if you don't like that content.
I agree to some extent. When I had livejournal, it was more authentic. Back then, you always have been seeing everything chronologically, and I hoped my friends will tell me about their lives. Unfortunately, writing is not for everyone, people subscribe to other users, not only their friends now, and algorithm feeds won.
Regarding social networks, though - people aren't exactly looking at your photos because they want to. Most social networks decide what you see, so the algorithm will put vacation photos in front of you even if you don't like that content.