confuse the defense. If they huddle, any substitutions have to be made while they are still in the huddle. Once they break the huddle, no more substitutions can be made by the offense, and the refs do not hold up play for the defense to substitute. The defense has to recognize the personnel while they are in the huddle.

If they don't huddle, the offense can run subs in and out all they want - but the defense gets a certain number of seconds from the last substitution to get their guys on and ready.

I'm sure defensive players are taught that if the offense subs late in the clock, they should just take their time to get out there. That (imo) is a valid tactic to potentially force the offense to use a timeout.