Capos can be misunderstood. In addition to being useful in getting different chord voicings using common open chord shapes, it is helpful when playing music that wasn't written for guitar with other instruments. When you encounter keys like Eb, B, Bb, C#, it can sometimes be challenging to play. On an acoustic guitar, barre chords can sound a little dull, so if you use a capo, often you can use the open chord shapes to get that clear sound AND get the proper chord.

It also allows you to move keys to better suit a singer.

I've heard some refer to using a capo as cheating. It's a tool, and if a tool can make your job easier, why not use it?

Another tip...let's say you are playing with a piano, and you have your capo on the second fret. You need to be mindful when communicating with the other musicians of what chord you are playing EXACTLY. If you play an open D chord shape with a capo on the second fret, you are actually playing an E chord. The pianist knows not of this capo thing you speak of :) ...so if you have to tell him what chord you are playing, tell him E, not D. If he tells you to play an A chord, know you need to play a G chord shape.