I can't say whether or not they are worth it, but they provide some degree of branding, mainly to "adjacent" communities. If I see a paper in IEEE something, I have some degree of assurance that there is a minimum quality level, even if I'm not that familiar with the area.

Also, at least for IEEE, they provide some help with running the conference, publishing the proceedings, etc.

Whether or not they provide value commensurate with their cost is a separate question.