People are very often not honest in real life as they don’t want conflict.
I think this is actually at the heart of MN and why it is even useful.
There is a greater inclination to honesty on here. For me, this is best illustrated by baby name threads, where, I think, people are usually genuinely wanting honest opinions.
I would never, could never, be as direct IRL if a friend asked me, so I guess that's why they come here: to know what people really think.
Other predicaments are harder. Sometimes with relationship advice etc people project so much of their own situation that the advice is honest but not necessarily objective. I guess there is no way round that.
But a very straightforward question such as "what do you think of this dress?" can generate useful honesty I think, even if a little spicily expressed!
I can only think of one time my post was deleted and it was for saying a heavily pregnant lady wanting to wear a very pink all-in-one jumpsuit to a wedding might risk looking like Miss Piggy. The Op got very upset indeed and had it deleted, but it was an honestly-given warning!
Sometimes, though, I think ops do get much-needed support.