Keys are very important in guitar (all all music) as thjey show us which chords go together. Just like foods- you wouldn’t put mustard on ice cream or chocolate sauce on your steak. They are the right ingredients but not put together as they should be.

So knowing which chords can be grouped together (in keys) is our recipie book for a musical meal. To find out which chords are in a key we need to look at musical scales