Not all software need to be speed critical but all softwares benefit from being faster. Part of the reason why we can afford to be less optimal when writing software is because many libraries have already been optimised for us (and it's not uncommon for these libraries to map to C or assembly code to utilise the hardware more efficiently), ranging from serialisation libraries to crytography libraries. This is not to mention the OS that we run the software on and other tools that the software depend on (such as databases). A small compiler optimisation improvement across the entire ecosystem can have a dramatic effect.