Hex wrenches (also known as Allen keys or Allen wrenches) are used to drive bolts or screws that have a six-sided socket. There are two styles of hex wrenches: metric and Imperial (also called U.S. customary, or SAE).
Why is this important?
You want to make sure you use the right tool for the right occasion.
An SAE wrench in a metric bolt will not fit properly. Neither will a metric wrench in an SAE bolt. Sure, you can use one that’s a teeny bit smaller, and angle it, but then you’re running the risk of stripping the bolt.
Here’s the thing – you can strip (damage) any screw or bolt by using the wrong tool to try and put it into or remove it from whatever it goes in. Strip it badly enough, and you need specialized tools to remove it.
A basic hex wrench set, in both metric and Imperial sizes, is a good recommendation for any tool kit.