Mainframe, MVS and zOS Discussion

I am learning assembler or say trying to learn. Assembler is difficult language to code. Since COBOL compilers can covert COBOL code into machine language, what is need of assembler?

There are things you can do in Assembler that you cannot do in COBOL. For example, if you want to access a data set for which the record length is not known -- assembler can handle this but COBOL requires the record length be known at compile time.