You'd learn x86-64 assembly to better understand how your high level code maps to the CPU. To see through the level of abstraction you're using. So that you don't do senseless things in your high level language of choice. Not to actually write assembly most of the time.

Although sometimes you have to write asm, if you need high performance. You can beat compilers, simply because they're very bad at autovectorizing and global optimization when control flow is too complicated.

Compilers are good most of the time, just not always.