as to why...

I submit that the potential subconscious cue of seeing "be civil" or "stupid sucks" by individuals in passing while going about their day, can in the aggregate, affect society in a positive way by nudging them away from unhelpful behavior. 

The invariably non-zero and non-adversarial effect is what moves me to enlist like-minded humans to signal their solidarity to other like-minded humans and gently nudge those who may not yet know they are like-minded humans too.  

it's as simple as that.