Guess it depends who you are blocking, where from.
When I found out about my ex and his OW, a friend said that oh yes, she had seen the OW "like" something of his on FB, and been suspicious but not said anything.
I have no idea how much people can see on FB if they are a friend of a friend, etc. so I just blocked my ex, the OW and anyone to do with them, in the hope that meant they wouldn't be able to see, for example, if I commented on a shared friend's post, or anything else. Not because I was raging with anger or doing it to hurt anyone's feelings, simply because I wanted to make sure I had some privacy.
At the time, I blocked my exh on Whatsapp for a while so that his status photos wouldn't appear; I did not want to see them. Unblocked him since, but the kids mentioned recently that he'd been putting loads of photos up on his status while on holiday, and I didn't see them, so presumably he must have blocked me somehow from that. I don't know or care how or why. I doubt it is to make a point of any kind. It's been years and we have nothing much to do with each other any more.
Other times, when very busy with work, I have blocked my bf on Whatsapp for a few hours just to make sure I don't keep checking to see what he's written, or feel obliged to answer.
In the past I have blocked people from OLD after a chat came to an end, or having not made a second date, due to the experience of people writing back after all to make rude comments. Not that I expected these people to ever write again, not because I was raging and fuming, but simply to avoid potential irritation.
People can still ring me up if they need something important; no big deal. I presume previous posters are thinking of other things when they say that blocking is immature.