Of course you could write most things in asm, but is it worth it? There's certain things which would take many thousands of pages of asm to write, and only take a hundred lines of code to do the same in a more appropriate or specific language for that task. It's all about the right tool for the job. You can write routines in asm that need a lot of optimization or write CPU specific instructions that need to do hardware operations(I/O), or even write critical system code that needs to perform very fast in asm, but otherwise you're most likely just wasting your time when you could be using to actually code what you had in mind in a much shorter timeframe.
Knowledge of the architecture your code will run on is however quite useful if you plan on writting efficient code, but not necesarry.