Not necessarily new optimization passes, but currently the order of optimizations is more or less fixed (you can technically rearrange them between builds but almost nobody does this AFAIK except that one Uber dev optimizing iOS app size). Even worse, there is no globally optimal order as what is good for one codebase might not be optimal for another. E-graphs can find the optimum arrangement for all codebases without requiring excessive amounts of memory and so could produce better output from compilers.

They won't find new equivalences on their own though. I doubt that is even possible, you can't in general prove equivalence of functions without also solving the halting problem. Also, even just defining the equivalences we know about are often only valid in certain circumstances that I doubt a computer could figure out in the general case.