Think of any photo as consisting of two parts
- the part of the scene that gets 'enough' light
- the part of the scene that needs more light
Most scenes consist of Type 1 only (think 'outside at average home'. Some scenes consist of Type 2 only (think 'indoors').
Some scenes consist partly of Type 1, along with partly Type 2 (think 'photographing outside my sunlit home, but including my wife in the garage')...your usual exposure would get the outside of the house just fine, but your wife would be consirably underexposed standing in a rather dark looking garage interior...and that is why you add flash use, to put light inside the garage.
Other scenes consist partly of Type 1, along with partly Type 2 (think 'my wife outside wearing a large brimmed hat that shades her face')...the part of her body in the sun would be exposed properly, but her face in the shade of the hat would be significantly darker...and that is why you add flash use, to put light into the shadow area of the hat.
The Canon dSLR in Av mode, with eTTL flash unit, is a very easy to use way of coping with the combination scenes consisting partly of Type 1 along with partly Type 2