Softboxes are designed to create flattering, but natural light for photographers. If you use a softbox between your light and subject it can lower the contrast and provide you with more control over where the light falls.
The size and the distance of the softbox from your subject is the most important factor in determining how soft your light can appear. A large softbox that is situated very close to the subject will create a light which wraps around the subject. This set up will make it easier to render detail in shadow and highlight areas.
Softboxes also provide a type of chamber between the light and the front of the box for the light to bounce in before it heads out in one direction (which is forwards through the front of the box). Softboxes give you a platform on which you can attach a fabric grid to further control your direction of the light. Some softboxes are designed to soften the light from strobes and not hotlights. A softbox for hot lights has to be heat resistant. However, a softbox for hotlights can be used for strobes.