Some people have a great ear for music and can write solid songs without formal training in music. Other folks come at music from the more theoretical side, although usually with a lot of implicit knowledge of and experience with music as well.
For most people who are not formally trained in music, their songs can be improved upon on a technical level by someone who has deeper theoretical knowledge (learned either explicitly or implicitly).
For a good discussion of this, check out Tim Ferris' podcast interview with Derek Sivers. Derek talked about how he had learned a lot about music implicitly. In one summer, a teacher of his formalized that knowledge so efficiently that he was able to test out of lot of classes (1.5 years worth?) once he went to Berklee School of Music.