I think that with increasing population of the world and lack of resources it's getting harder to live and our brains have to function better, faster, more stable with every next generation. But we are no computers and our brains don't function equally. Everyone of us has better and worse sides of our minds. When the bar goes up more people cannot keep up with expectations. And that's why they need someone who can find the reason of their deficiencies, make them aware of it and will tell them how to overcome it. Such person is a psychologist.

@My Han, you said that psychiatrist is a kind of better skilled psychologist. I don't know how it is in your country and in other countries, but in my country these two professions are trained separately and have different approaches to mental disorders, though their fields of interest overlap. First of all psychatrists are doctors, what means they have passed all the way of medical studies and speciality training like every other medical speciality. The should have at least general knowledge of human diseases and used medications, so they can find the medicine which not only helps patient in his mental disorder but will cause lowest possible harm on his general health and will not interfere with other medicaments he takes. Their aim is to diagnose and cure mental diseases. They sometimes use psychotherapy which is complementary to medication, bu they usually need some additional studies to use it correctly.
The psychologists have separate university studies. They are trained to deal with mental disorders (not diseases) and help people using the psychotherapy. When you have some problems with you, your past, the community you live in, and many more difficult life situations - the psychologist is the best person to help you (you probably wouldn't like to go to psychiatrist with this kind of problems).

But these specialists often work together on serious cases.