If you're facing a barrage of attacks from the other team, you can stop the game, reshape and interrupt their momentum. If a team is chasing a one goal deficit and they throwing the kitchen sink at the other team, then the team in the lead can use their 5 subs to interrupt their momentum. Moreover, the defending team will be more likely to incur yellow cards, more players will be willing to take a yellow because they have the security on a extra 2 subs.
I'm not against this change in general, but putting it in the same season where you had 3 subs means that you have altered games within the same competition. Would an extra 2 subs change every game? No, but there are many games which would have been changed by it.