I am new to MMA (3 weeks) and I wondered if I could learn to box without a trainer. The answer is no. Trainers can see your weaknesses and help you eliminate them, for me I think the two most important tools a trainer provides is breaking bad habits and building my stanima. Both of these are hard to do on your own but breaking bad habits is impossible if you can't recognize them.
As an example I tend to reach and strike with with my arm rather then my body. Working out by myself I may have never noticed this but when (25% of the time and increasing) I strike correctly I can feel the cross throughout my body and I love the sound it makes when I hit the bag. But without a trainer I would still be reaching, losing my balance, and striking with my arms rather than my body.
One more thing different trainers have different perspectives and can help you with different things. My usual trainer is great but other trainers have a different perspective and a different teaching style that mixes things up and helps me progress.