Because, 91/30's were sighted in with the bayonet on at the factory.
http://elby.yolasite.com/
Tried it with bayonet on and off. No difference in POA/POI. Don't waste your money. YMMV

Or do it the Russian way Hammer and Punch....Problem Solved I did it with mine Took a little time but better than spending money on a specific tool i used once.....