When the offense substitutes by rule play is stopped until the defense has a chance to not only react but put players in place.
With hurry up:
1. Offense steps onto the field.
2. Defense sees what offense steps onto the field.
3. Defense sends player package onto field.
4. Oops, that wasn't the best player package after all.
5. Offense realizes this, doesn't substitute, goes hurry up, and attacks the mismatch.
6. Defense doesn't have time to substitute and can't adjust.
Without hurry up:
1. Offense steps onto the field.
2. Defense sees what offense steps onto the field.
3. Defense sends player package onto field.
4. Oops, that wasn't the best player package after all.
5. Offense realizes this, but ho hum twiddly dee let's take 20 of the 25 seconds we have to snap the ball and just stare at coaches waiving their arms on the sidelines.
6. Defense has time to substitute and is only punished for that one play.