Flash allows you to add light to a scene. There are many reasons for doing so, such as
1) Available light quantity sucks (too dark)
2) Available light quality sucks (ugly puke green fluorescent or sodium vapor)
3) Available light harsh, or coming from the wrong direction (sunny day)
4) To equalize exposure of subject and background (shaded subject with sunny background)
5) To make your subject brighter than the background
6) To freeze motion of a moving subject
Flash also has its drawbacks. It limits your shutter speed, can be distracting and annoying to subjects, requires recycle time between shots, and may be a different color temperature than the ambient light.
The ability to add light to a scene makes the difference between capturing an image and creating an image. It can take your photography to a whole new level.
As you venture into the world of flash, just remember that every flash photograph is two exposures in one: An ambient light exposure and a flash exposure. You will need to learn to think about the two exposures separately and learn to manage both.
Its an exciting world, but there is a serious learning curve.