How psychology interpret Self-talk?

When I run within park, I see a guy around 40~50 talking about how stone staircase should be placed, and discuss about project's issue, but he was alone and not talking to any mobile phone and hand-set.  He is talking to himself, he behaves normally as an normal people, just talking to himself, this type of people can easily be found within city. Sometimes, I talk to myself too within private place.

Does anyone have any suggestions on how to interpret self-talk behaviors from psychology's point of view?
Thanks in advance for any suggestions