Cracks allow water to get into the pavement, which causes your asphalt to deteriorate at an accelerated pace. If the water reaches the sub-base, it will weaken your pavement structure and cause alligatoring and potholes - which are more costly to repair.
Crack sealing prevents water from getting into the pavement, to prevent or delay pothole formation, and should be done at least annually.
New cracks that have formed or old cracks that have reopened, between ¼” and 1” wide, should be filled or sealed with crackfill as soon as possible.