If you're on a general purpose PC/smartphone with packaged software then the OS vendor specifies a base set of extensions that everything must implement -- for Linux at the moment that is RV64IMAFDC aka RV64GC.
All of those extensions (except maybe A) are very generally useful and pervasive in code.
Some other extensions, such as the Vector extension, will provide significant benefits to applications that don't even know whether the system they are running on has them -- you'll just get dynamically linked to a library version that uses V or doesn't, as appropriate.
To take a very trivial example, on a system with V, every application will automatically use highly efficient (and also very short) V versions of memcpy, memcmp, memset, bzero, strlen, strcpy, strcmp and similar.
The same will apply to libraries for bignums, BLAS, jpeg and other media types, and many others.
Find me a processor that supports SSE4 but not SSE3. That's the problem. With x86 you pretty much can say "we're targeting processors made after 2010" or whatever and that's that. You make one binary and it works.
RISC-V allows a combinatorial explosion of possible CPUs. You can have a CPU that supports extension X and not Y, but another one that supports Y and not X.