It also allows you to draw a card so you are benefitting from increasing a Strength stat AND replacing the card you just played
Finally, you can mitigate the damage you take as it comes via creatures like Souls Attendant or Essence Warden, or gain a bunch of life through cards like Exsanguinate and Debt to the Deathless to offset early losses
Suddenly your 4/4 is a 6/6 that gives all of your permanents hexproof - which is absurd on its own - and then also puts two +1/+1 counters on each of your creatures at the beginning of combat on your turn
For opponents, one could force them to draw on a full hand, meaning they'd have to discard all three cards
To show how efficient this card is for the right deck, the final versions of Miracle Grow, which were dubbed Super Grow, used Mystic Enforcer as a finisher, since by the time you'd get around to casting it you had an all but guaranteed 6/6 flyer for four mana