I taunt when I knock people down, because when Mila jumps to the next level of the stage, she says "Lets go!". In her taunt she says "Alright! Lets go!". I don't let the taunts bother me though.
People use taunts as it's a strategy to evoke anger in their opponent. This leads their opponent to play more aggressively, and will in turn make them more vulnerable to any form of punishment. Other people use them because they are dicks and like to piss people off.
Personally, I don't use taunts because it's childish in my opinion. I'd rather have a fight involving no taunts or forms of overkill. I see this a sign of reciprocal respect, which I think is a good thing.