I agree that wanting to have more than 32bits of addressable space (not 32GB of memory) in a web app seems excessive.
However the real win is the ability to use 64 bit types more easily, if for nothing else other than it simplifies making wasm ports of desktop libraries.
It goes beyond that. Many languages expect that you use types such as size_t or usize for things that are conceptually collection sizes, array offsets, and similar things. In some applications, it's common that the conceptual collection is larger than 2^32 while using relatively little memory. For example, you could have a sparse set of integers or a set of disjoint intervals in a universe of size 2^40. In a 64-bit environment, you can safely mix 64-bit types and idiomatic interfaces using size_t / usize. In a 32-bit environment, most things using those types (including the standard library) become footguns.
I work in bioinformatics. A couple of times a year I check if browsers finally support Memory64 by default. They don't, and I conclude that Wasm is still irrelevant to my work. I no longer remember how long I've been doing that. Cross-platform applications running in a browser would be convenient for many purposes, but the technology is not ready for that.
One could argue that size_t should be 64 bits on wasm32 since it's a hybrid 64/32 bit platform (and there's the ptrdiff types too which then would depend on the pointer size), but I guess sizeof(size_t) != sizeof(void*) breaks too much existing code.
However the real win is the ability to use 64 bit types more easily, if for nothing else other than it simplifies making wasm ports of desktop libraries.