Assembler generating was long time a standard feature of compilers simply because writing the assembler code was easier than directly runnable or linkable object code. And for language designer and compiler programmer it was easier to read verifying correct code generating that way. Why bother writing assembly compiler, linker etc. when you usually already had them in the system that time.

It's easier to directly generate binary object code than assembly. It also makes for a way, way faster compiler. Also, one of the first programs I wrote when developing my compiler was an object file disassembler.