What actually makes snipers defence heroes?

This isn't ment to be that big of a question, but I've been thinking about that since Hanzo and Widowmaker are best suited to fight against those type of enemies that are either slow or most of the time completely static when doing their things, that is if the sniper player is skilled enough.

One of my friends who has lot of playing time with Hanzo once said "Hanzo is actually better in attack, since he works better when enemy team has Bastions and Torbjörns, not Tracers." This seems to make an interesting point about how in attack the enemy is moving in fast phace, while in defence the enemy is more static, and snipers would feel in general more effective against the latter type.

Or hero roles are only titles in this game and I'm just looking too much into it.