There’s actually a way in which a capo makes it harder to play. For the most part, one uses a capo to play standard chords/shapes in first position. However, if you’re playing (for example) in the key of D Major with the guitar capo’d at the fourth fret (resultant key is now F# Major), when you go to add fillers, licks, stabs, lead lines or a solo up the neck, you need to mentally switch over to F# Major. Soloing over the changes in D while everyone else is playing in F# Major is going to make you feel like a total dork.
The group classes I’ve been involved with teaching (Guitar from Scratch, Sing Along with your Guitar), we spend a lot of time finding the optimum key for the largest number of people to sing well vs. the easiest guitar parts to play. As simultaneous section leader/lead guitarist, I have to play with the capo on so everyone can see the capo position and follow my hands for the chords. Then when I take the solo, I have to remember, sight-transpose, write it out or go home feeling very, very stupid…
We can only guess as to why she used a capo. Among the possible reasons are: her guitar skills are limited to where she could more easily play in the lower (capoed) key than the actual key; her finger reach is limited to where it’s easier for her to make chords with the shorter fret-to-fret distance higher up the fretboard; the songs were in an odd key (e.g. Ab) where almost anyone would have used a capo to strum rhythm; she/they preferred the bass note layout of the capoed key; and she/they preferred the higher pitch afforded by capoing. I do find it odd that she would use a capo on every single song.
You could use it as a crutch, but it won’t get you that far. You can put the capo on the 1st fret and make your F and Bb chords easier. You can put it on the 2nd fret and do the same with F# and B chords. Beyond that and you can get away with first-position chords.
Using a capo (or tuning down) to adjust a song’s key to match a singer’s voice isn’t cheating either. That is just arranging a song for a performance.
As others have said, the capo is primarily a tool. It allows you to choose both the key and the chord forms. Without it you might have to choose one of those over the other. Often you want the notes that come with a G-chord form, but you want them in a different key. A song normally played with the capo on the 5th fret using first-position chords can also be played without a capo and different first-position chords, but the song sounds different – it is missing the high notes.
Finally, a capo can be essential for hammer-ons, pull-offs, and ringing notes as well. If you want a note to ring while you pick out chords in a progression, then it will often need to be an open note.
