Do I need mulch in my garden? Why? What does it do? What kind should I get? How much do I need?

I am in Berkeley, CA with a combination of flowers and vegetables in my smallish garden. Whenever I walk through the neighborhood I see yards where it looks like the folks know what they are doing, and there is always mulch around the base of the plants, which makes me think I should get it, but I don’t know the reasoning behind it, how to choose a mulch, or how much to get.