There is an argument that knowing machine code will help you understand what the machine is doing in more detail. But so would knowing how the transistors work and nobody suggests you need to know semiconductor physics to be a good programmer!
Having some idea what the code is doing and what operations, such as copying memory or accessing a disk, are expensive is useful - but you can know these without being fluent in assembler. There was also an argument that assembler helps you debug a stack dump - but modern compilers can produce some very odd optomised code which is almost impossible to understand/.
If you want to understand assembler for the pure joy of it - then there are a number of synthetic assemblers, or at least more modern CPU designs which are cleaner than x86