>

I don't know for sure, but they offer Russia 2 things.
1) More bodies to throw in the fire.
2) Some kind of made up legitimacy "look, another country doesn't like Ukraine, so we must be right"
It's like they they want their excuse to enter the war to be as flimsy as possible >

It would damage Russia to let Belarus to enter the war. >

They, (Russia), could blame Belarus for "incompetence" if something else goes wrong.
Plus they would have more cannon fodder to throw at Ukraine ... >

Belarus has nothing to offer in terms of war