This makes you run fast. Should I say it again? The Speed Booster makes you run fast. It boosts your speed! When you're running fast, you can run over crumbling bridges before they crumble. You can also break through walls with little green arrows on them. It rarely has any use in combat or anything, it's basically just used to solve puzzles. Appears in Metroid Fusion, Super Metroid, and Metroid: Zero Mission.
This upgrade also gives Samus the infamous Shinespark ability, which can be used to traverse theoretically infinite vertical and horizontal distances, as well as destroy enemies. Performing this move will also allow Samus to smash through blocks that are weak to the Speed Boost. Last but not least, the Shinespark can be performed while in Morph Ball mode.