When you start playing covers, or when you start leading worship using music other people have written (yes, that means you, Chris Tomlin), you quickly discover that nearly every songwriter on the planet except for Stephen Merritt sings in a register that you can't possibly reach. The solution? A capo. To this day I'm thankful for the engineers at Shubb, G7th and others. I would make a lot less guitar-accompanied music without them.