Yeah they can be used to make songs easier to play on the guitar.
That drew a laugh from me because I think I am the only person in the world who can't use a capo. My brain is so locked into the fret markers, chord structure based on those fret markers and sight reading that I can't make myself think "This is just like playing it in C. Transpose everything the right amount of steps needed and play it like you are playing in C." But my eyes then tell my brain "Nay. Nay. This is in E, and in E that minor 6th is a C# minor, and we know how to play an E# minor, right? Now put your barre on that 4th fret and play it like you know how."
Which is to say that with a capo I have to think too much about "What chord is this relevant to C?" and when you have to think rather than just play by instinct it's pretty much a mess. I have tried many times and without charting it out in "normal" (written) key and then taking another sheet of staff paper and transposing it, I just can't use a capo. I DO have something coming up that has a persistent, repeating part, and to capo the 3rd fret is the only way to play it where open strings can resonate. (The part calls for G C G D (held), and if I capo on 3 I can play a fingered 3rd and 4th string and an open 1st and 2nd string and get what I need.)