Obviously compiling for the target architecture is best, but for most software (things like crypto libraries excluded) 95% of the benefit of AVX2 is going to come from things like vectorized memcpy/memcmp. Building glibc using ifuncs to provide optimized implementations of these routines gives most users most of the benefit of AVX2 (or whatever other ISA extension) while still distributing binaries that work on older CPU microarchitectures.