Well going back prior to MPLAB, I wrote a version of WinPicProg that included a simple BASIC compiler, which compiled to MicroChip Assembler, and then called MPASM as a command line application to create the HEX file which was then programmed in to the chip.
So essentially it's so you can use it from within another application, not for manually typing in all the options any running it directly. It was quite common practice to provide command line options in DOS programs, even those which had their own IDE.