I don't think the question was worded well, since I don't think the crew were ever referred to as "red pills". I think Cypher referred to people within the Matrix as different pill colors (red/blue) to describe those that were oblivious and wanted to stay that way, and those that would want to know, and could accept, the truth, if it was presented to them. To answer, we'd need to decide if we are talking about "red pills" as in, the consciousnesses of the crew, or the pills themselves.
If asking about why the "red pills", themselves, entered the Matrix and why they exist, they are programs set up to hack the Matrix and allow a person to be found and freed - probably written by Trinity or other human hackers. That's why after Neo takes the pill, you see Trinity and the crew working to locate Neo and unlock the "battery cell" Neo is in - we can infer that the pill has something to do with that process during that scene. When Neo wakes up after his acupuncture to restore his muscles, he asks about the pill and they confirm for him that the pill was a program used to find and free him.
As far as why the crew went into the Matrix - more than just to free the enslaved and, my guess, for intelligence gathering on what the machines might be doing - was, at least in the first movie, to:
- Find Neo,
- Take him to see The Oracle, and
- Free Morpheus.