First of all, many, if not most, computers run software so trivial on microcomputers so small, installing a compiler for them would just be _way_ more effort than to just code to program in C. Typical examples are the microcontrollers in electric toothbrushes or other smaller embedded systems. It's hard to get your software running on a 4 bit microcontroller as modern C makes assumptions like having your memory addressible at a byte level.
Then there is another, in my opinion more important, point. Assembler is a great teaching aid. It shows you what the computer actually does. Understanding things like pointers is trivial in Assembler so you can learn a lot from it. Also in Assembler every control structure hurts, as you need to keep track of it yourself. This nudges you towards writing simpler code, away from thousands of nested if statements and functions with hundreds of lines. Those are desirable traits in all programming languages.
No language is suitable for everything, but most languages have at least one are where they are really useful.