Membership in the IEEE and ACM is sometimes portrayed as a sign of professionalism. But all that is involved, as far as I can tell, is sending them your money. In return, besides the potential resume line, these organizations sponsor conferences and journals.
I can always attend a conference or subscribe to or submit a paper to a journal, whether I am a member or not. If being a member makes some of that cheaper, or is a prerequisite for admission then OK, but I still don't see the purpose of these organizations.
The answer, as far as I can gather, is that their most important value is to provide some reading material. I'd suggest that this is not worth the money given the wide availability of other valuable reading materials.