I'm frequently asked, "why have someone else master my/our work?" Well, there are several reasons why I would suggest a separate mastering engineer. Remember, it's not financially advantageous for me not to master an album I've mixed. In most cases, I prefer to send albums I've mixed to a different mastering engineer to get your music sounding as good as possible.
Firstly, you, the artists, have worked on your songs from the writing stage and have probably been present throughout the whole recording and maybe even the mixing stage. Mixing also takes time, so many mixing engineers are also over-exposed to their mix by completion. When we reach that point, our ears are so used to hearing the mix that it can be hard to listen to it objectively. Objective listening is a considerable portion of the mastering phase, as much of the work is "fine detail" changes. This is the point where a fresh set of ears comes in useful.
On top of the objective listening, a lot of mastering equipment differs from the equipment used in mixing. A mastering studio is equipped for a specific job that a recording/mixing studio might not be equipped for. A mastering studio will also have different monitoring. Particularly in cases of mixes done in home studios, issues caused by frequency ranges that your home/mixing studio monitors didn't reveal would be more evident in the mastering studio. Things you may not hear could become a problem later on!