Assembler vs Hex Editing

Time for my semi-monthly dumb question.

What does an assembler do, and more specifically, how does one use an assembler to either implement others' custom code or insert one's own?

Is there a difference between using an assembler and generating the opcodes via changing the bytes in hex?

Nearly all of the assembly tutorials point to learning the assembly language, and while obviously, that's required to understand the code...I feel like something is being skipped.