Why does your mind wonder?

Well your mind doesnt have legs.

Ans 2.
In the event of a perceived threat, or opportunity, the mind concentrates. The human mind evolved that way because it gave a competitive advantage over those without that mind-set.

When there is nothing that has to be done, the mind wonders or wanders. This also gives a competitive advantage to those humans that do it, and especially to those societies containing at least some people who do it.