What is the point of researching Heresy?

Hi to all. i just wonder what exactly Heresy is for. I mean i know that it is killing opponent's unit instantly instead of converting.

The thing i really want to know is : Isn't good converting that unit to your side rather than kill?

For Example: If there are 10 Teutonic Knights in your Enemy's army and you had 5 Monks with Heresy probably the winner would be Teutonic Knights right ?. Maybe a few of TK killed at worst.

But if there are Monks without Heresy, They would convert the half of the army and the converted army would kill the rest. Because after converting phase the Monks would start to heal his army.

What is all the point of this? I can't get it