When your partner has rebid 1NT and you have five cards in your major, you may still have a fit, but you can't be sure that you do.
With a weak hand, you can bid your major at the 2-level. But with 11 or more points, you can use New Minor Forcing.
Bidding the other minor at the 2-level shows game ambitions and asks partner if he has three cards in your major.