I think of ambient light as "the light that's there before you add any lights." Years ago, when raytracers only made hard shadows and there was no GI, you had to put some ambient light into every scene or your shadows would be dead black. It was basically poor man's GI. So ambient light is definitely not necessary when using GI, although you can use it if you want, of course. Overall ambient light has the effect of reducing the contrast of a scene, because your ambient light setting defines how dark the darkest shadow can be. If your ambient light is a medium-dark grey, then your darkest darkest shadow will be that same medium darkness.