I have never read the books, I've only seen the first two movies. I've wondered about what the imprinting part that jacob had to do and why it had to happen?

He didn't have to do it.

It's complicated to explain. It would be a lot easier for 당신 to read this page:
link

Imprinting is not something that 당신 have to do it just happens out of nowhere.It's a connection and a feeling that a 늑대 has that cannot be controlled. Some 늑대 imprint and some don't.
Jacob went to kill the baby when the imprinting happened. I suggest 당신 watch all the 영화 to understand.

Basically, it happened in order to close off the 사랑 삼각형 fully, and to give Jacob a happy ending instead of having him mourning Bella for years. It also created peace between the 늑대 and the Cullens, and made it impossible for the 늑대 not to be fighting alongside them at the end.