Never will understand why people celebrate being able to create an echo chamber around themselves. Enveloping them in a place where their opinions are mirrored back at them and are always 'correct'. Where they are never challenged to think critically, never having to argue a viewpoint and perhaps learn something new - god forbid having to change one's opinion as a result!
you answered your own question: for some people a discussion forum is a 'place to be', and they want that place cozy and clean. it's just a function of the inordinate amount of time they spend on the forum (which reinforces attachment and entitlement) and their tolerance of noise and dissent. yes, it looks like an addiction because it is one. a soft one.
other than that it's not so strange. look at google, facebook and even the media, it's all about reinforcing one's own ideas (or spreading dogma) and feeling cozy in that slice of 'reality'. this very forum is moderated. it's quite tolerant moderation in comparison to others but still threads get closed, people even demand threads getting closed! it's the same thing at a different level.
the bottom line is that humans are wired by evolution to meddle in groups around 50 individuals, knowing each other personally, yet we live in a planet interconnected with billions of strangers ... and it works! but our human brain is just not there, unable to cope with that much diversity and information. so we seek safe havens where we are familiar and where we our beliefs are not constantly challenged, but reinforced by the ... tribe. our 'place'.
now, there is another aspect to blocking, and that's making it public. why would you? this is a clear call to 'likeminded' to do the same, help to keep the 'place' clean and ignore collectively any disturbance so to not only 'hide' unwanted content, but eradicate it. this is an agenda. of course it's a fantasy, doesn't work ... and can actually fire back and make groups fully endogamic and finally stagnate to extinction (although i don't think this will happen to this forum anytime soon, there's just to much movement. still).
on a personal note, for example i do spend (way too) much time lurking lately, because reasons. i'm aware and while i'm 'here' (won't be forever) i love reading (or skim reading) everything: interesting, fun, odd, antagonizing, informative, offtopic, pathetic ... it's a quite diverse group of people and it's both entertaining and instructive. but the very moment i felt the need of filters or blocks it would raise a red flag, a sign i was getting sucked in way too much.
i have actually blocked one (1) user, and it was more of a pun block than anything because of a very arrogant statement (about blocking other users, coincidence) in one single post. one of many interesting and informative posts because it's actually a smart and informed user, but that block stands as a testament, and there will be no more. i'm telling myself i don't care enough about this 'place' to block anyone, nor to bother about removing a block. i care about people, ideas, opinions, ... not 'places to be'.