Importance of ACM/IEEE in TCS conference
One thing ACM brings to a conference is name recognition: if I see that a conference is ACM-sponsored, even if it's one that I haven't heard of before, I can be confident that it's reasonably high quality and well organized, rather than being one of those scam conferences that accepts everything and profits off the registration fees. (I'm less sure of that with IEEE after the Schlangemann scandal.) Of course, there are also good conferences that are not sponsored by these societies. And I wouldn't want to rely on this halo effect for making decisions about tenure or anything important like that — better to solicit opinions from people who know that area better — but it can be useful when deciding where to send a paper.