When and why to use Speedgrade?
You ask what it does: Speedgrade is a color grading / correcting tool. Davinci Resolve is another, and there are others still. They allow you to adjust the characteristics of a video on a shot-by-shot basis.

It's never necessary to use any color grading tool, but you'd do so when you want to match scenes from different sources or shot under different conditions, or to give your video (or one portion) an overall look. The usual use is for consistency.