If you want to protect your lumbar spine during load bearing activities that stress your spine, it's probably a good idea to learn to brace your core. Your erectors help to a certain extent. If your goal is to get strong then it will only help to recruit more muscles.
It isn't really necessary during leg curls in terms of safety. But if you're holding on to something and creating tension throughout your body helps to lift more, then why not try it?
I have gone years without learning to brace properly when I started lifting. My abs did not adapt and learn to maximally contract itself - I had to focus on it for a period of time until it became automatic when getting tight. It's possible to get stronger without bracing properly but you lose out on strength potential and it will become a limiting factor at some point.