To answer your first question, it's completely variable on the company and their current needs. Some may need providers of any kind, others may only need psychiatrists and say that they are "full" for masters and/or doctoral level providers. On occasion, even if they're "full", someone with a true specialized niche (ie gambling treatment, eating disorders) may be able to get on if they can plead a good enough case.
As far as supervision, LPCs and LCSWs typically do not supervise therapists, they supervise MAs and MSWs as they learn to become therapists. There's a big difference. It's a mandatory requirement of every licensing board that license-eligible practitioners receive a certain amount of clinical supervision. What is done will vary based on the supervisee's job description, but generally there should be oversight of and responsiblity for their clinical work, continuing their clinical education, assessing their strengths and weaknesses as practitioners and addressing those issues that could interfere with their ability to practice ethically and effectively.
As to why someone would supervise, I can only speculate that for most it's a recognition of the importance of quality education and training in [insert field] in order to promote that field. Beyond that, some may do it for the above reason and because they can charge for it, and others who are agency-based may be told by their supervisors that they'll be doing it.
