I think I had an aha! moment last night when I couldn't sleep. Lets use the tune Star of Munster as an example. I learned Star of Munster in A-minor. It involves playing notes on strings 1, 2 and 3 of my 4 string tenor (no notes fall on the 4th string/G-string in the key of A-minor). I've never been able to play this tune cleanly because several times during the tune you must play the notorious 7th fret B note (on the 1st string/E-string). So, what I've done is move the whole tune "down" a 5th to D-minor. This means that everything I was playing on strings 1, 2 and 3 is now played on strings 2, 3 and 4. When it comes time to hit that 7th fret note, because I have one more string to use in D-minor, I can play the open E-string and get that note. The problem is that since most folks play this tune in A-minor, I would not be able to play with them if I'm doing it in D-minor. Here's where the aha! moment came in - a capo!!! I've never played guitar, and never used a capo, so I've never really thought about them before. But, theoretically, if I used my D-minor fingerings but put a capo on the 7th fret, I should be back in the key of A-minor for Star of Munster. So I think I'm going to get a capo and try this out.
In addition, the main chord shapes of G, C, D and Em are very easy to play on the tenor banjo in Irish tuning. However, I don't always like the chord shapes of A, D, E and F#minor for example. So, another use for the capo would be for a tune that's supposed to be in the key of A. You could capo on the 2nd fret, and then play the tune and/or chords as if it were in the key of G, but it would sound like A. It seems like that would make things a lot easier.
A guitar player reading this would probably be like "no duh", but this is the first time in 4 years of playing that I ever thought about this. Now I'm going to look for a capo!