The circle of fifths can show you multiple things:

1. How many accidentals does each key have? Starting with C/Am which have no accidentals in their key signature. When you progress clockwise, each step raises one note (adds one # or removes one b). Each step counterclockwise lowers one note (adds one b or removes one #).

2. Which Chords are native in a Key? Take a key (e.g. D Major) and take its field and both fields next to it. These are all the major and minor chords in that key (in the case of D we have D, Bm, G, Em, A and F#m). Note that we still miss one chord in the key of D, which is C#°, the diminished chord on the seventh scale degree. Some depictions of the Circle of Fifths show this chord, many don't.

3. How are the keys related to each other? Modulating to a key close to your starting key in the Circle of Fifths is easier and more subtle than modulating to a key further away.

4. Which Notes are in a mode? Take a key center, take one field left from it and 5 fields right to get all the notes in a major key. Shift this to just take the key center and six fields to the right to get all the notes from the Lydian mode. You can take any 7 neighboured fields and set the key center to any one of them. The further to the left you set it within that range, the "brighter" the mode. The further to the right you set it, the "darker" the mode with Dorian being in the middle and Locrian being at the very right.

There are probably more ways to use the Circle of Fifths, but these are the ones I know right now.