The “Validate” option is only available for the question's author.

By validating an answer, the question's author states that the latter is the one that does answer his question, in respects with his expectations.

The validated answer is not necessarily the highest rated, although there may be an inherent correlation between votes and validated answer, in the sense that in many cases, the expected answer coincides with the most voted one.

A question's author should seek to validate an answer as often as possible, since it shows that he is satisfied with what he got, and therefore, there's no need to go further from its point of view. In a way, it shows that the question is “resolved”, although not necessarily closed.