anubis B main smoke

[Frezmo]

in almost all of my games players on CT side will protocol smoke off B main every round.

What is the purpose of this? There has got to be a reason as to why it is good to do that otherwise people wouldn't do it on protocol.

What's the reason?
I find that often T players can sneak to the right-side of main behind the smoke which can be kinda annoying...