Why you can't use blockbullet in a payload instead of nodraw?

This was in my head for a long time and since can't check it now i wanted to ask it.

So in payload maps or when making a train people use nodraw (i suppose it's because blockbullets can't be turned into a func_tracktrain) but i wanted to know the whole reason.

 

Nodraw also blocks NPC line of sight, so that'd include things like sentries iirc. Just to make sure no awkward bugs occur, I presume

 

Thanks