The use as semantic stop sign comes from the fact that if something is obvious it does not need to be stated.

There are times when it's useful to state things that everyone present already knows. Most of the uses of "it's obvious" fall out of these cases.

For example, the rules of formal argument in an academic context might normally require each step to be given in full; but for pedagogical purposes it's not always useful to do that. The phrase there means you're temporarily lowering the level of rigor to make the argument clearer or to save time, and that's a useful thing to have in your toolbox.

Alternately, if you're making a complex argument with a lot of moving parts, salience issues often come into play; your audience might need to be periodically reminded of certain simple facts, but stating them without qualification might be viewed as condescending. "It's obvious that..." in this case serves as a social lubricant, a way of saying "I know you know this".