Assembler is used to code things like the initial boot sequence of a computer, before the runtime environment needed for a higher level language is available.

Assembler is used to code the interrupt entry and exit sequences in the OS, because you need exact control over the contents of the stack and registers.

Assembler is used to code instructions that are not generated by a high level compiler. These are things like reads and writes to processor control registers and machine specific registers.

Assembler can also be needed to code certain twisty interlocked instructions for which compiler intrinsics are not available.

Most of the time, you can use C, which has a pretty good “asm” feature.