From a beginners perspective, the capo allows you to use the simple chords you know and have learned to play songs with chords you haven’t learnt yet. A capo on the 2nd fret changes the “D” chord to an “E” Chord.
The “C” Chord becomes “D”, “A minor” becomes “B minor” etc etc. This can simplify things opening up the number of songs you can now play!
It also allows you to easily transpose a song. Now there’s no need to try and sing notes higher or lower than you actually can. You can take this song and bring it into your or your singers range.