Comps are the comprehensive exam that the Master’s progam I’m in requires for graduation, if you do not write a thesis. It’s actually a university requirement for non-thesis programs. Some ALA accredited Library Science/Information Science (whatever the school calls them) have neither a comps or thesis requirement.
It’s lowercase because that’s the way we wrote it. Some will capitalize, some will lowercase. No difference.
That can be a loaded question. Someone can become a librarian without the MS (MLS, MSIS, again, depends on what the school calls it), but it can be more difficult to get jobs. I decided to get the M.S. because this is a career change for me, and I know that getting a decent paying job is going to depend on having that degree.
There’s a woman in our program now, though, who is actually the director of a nearby library system and she is just getting her M.S. I don’t consider her any less of a professional than the women who’ve had there Master’s for years and work the reference desk in the university library. But some would call her a paraprofessional even though she’s in a professional level position.
It helps people in the field see you as a professional, and it provides some basic knowledge of the field. Not absolutely required, but helpful.