Is a tool necessary to bend a brake line?

Yes ... unless you are very careful and patient.

It is very easy to kink (accidentally create a sharp bend or fold) in a brake line when you are trying to bend it. There are several types of hand tools, reasonably priced, that make bending easier. Typically, they work by bending the line around a round, solid core, so the line doesn't have an opportunity to kink.

By the way, if you're bending a brake line and it kinks, NEVER try to un-kink it. The original kink has already weakened the line, and bending it back will weaken it more.

Unless you're an experienced do-it-yourself mechanic, it's probably better to leave brake work to the professionals. The consequences from bad brakes can be tragic.

Possibly if the bend is acute or you are inexperienced.. I bend brake lines up to 90 deg by hand with no ill effects.