3...Qb7+ 4.Qf3 This is the idea. After the queen trade, the c6-square will again be accessible for the knight.

Guaranteeing an outpost on c6 for the knight. From there, it will block the c-file and there will be some Ne7+ ideas in the air.

A natural move to protect our pawn chain.

Botvinnik is not interested at all in the a-file. The plan is Nc4-Nd6, with a very active position.