My Lands: Black Gem Hunting

since this spell also helps the enemy how is it useful?

It's useful because if you find units that yours can pursue, without being victims, then you can do extra pursue damage and really wipe out some standing troops.

Say you find a base with fliers and foot soldiers.
Then you can bring archers and cavalry to pursue and cast Total Pursuit to really pursue and defeat the troops.
This gives you all the pursuing power and none to your enemy.
See how it can help?