(Disclaimer: Capo can be beneficial to any type of musician, but for the purposes of this post I'm going to refer to its use with guitar players. However, its features will apply for other instruments too.)
Firstly, and probably the most important feature of Capo, you're able to customize the tempo of a song without effecting the pitch, which basically means you can slow down a song to a specific tempo and you'll still be able decipher the notes as well as play along with the song in the original key. Of course there is some degradation in the quality of the audio, but this is minimal and expected. The point is it allows you to hear each and every note and decipher how those notes are being played (is that a bend or a slide? for example). You can then play along at that speed, when you're ready, move the tempo closer and closer to the original until you're finally able to play along at normal speed. If you want to get super good at playing the part, increase the speed above normal!
Another important feature of Capo is Pitch Control. Though not often, there are times when it's useful to change the pitch of a song. For example, say you want to learn a guitar part in a particular song, but that guitar part is in Eb because the guitar has been down tuned a step. You could down tune your guitar to Eb, but that's a bit of a hassle. With Capo, simply move the pitch of the song up a step and the song will match the key of E.
Capo also allows you to loop sections of a song so that they play over and over again. This is an ideal way to practice and perfect a tricky or daunting riff. And of course, you can apply the tempo and pitch control to the loop, too.
Three effects are also built into Capo to help you hone in on what you're trying to listen to. A mono effect, a voice reduction effect and a 10-band equalizer can be applied to zero-in on the instrument or part your looking to focus on, so that everything else takes a step into the background.
Finally, Capo's spectrogram brings a visual cue to what you're hearing as well as illustrating where you are in a song with a time frame and the use of markers. The Drop Chord Marker (chord detection feature), though not perfect (it's impossible to un-bake a loaf of bread), helps you identify chords and notes on the spectrogram, and a tab feature helps you transpose those notes into tab music, all in one place.
Best of all, Capo is really easy to use. Simply drag any track (WAV, AIFF, MP3 and unprotected AAC) from your iTunes library into Capo and you're ready to hear and learn with Capo's features at hand. I can particularly see Capo being extremely practical and useful in a music lesson context between student and teacher.
Although these features can be found in other pieces of hardware and software, I've not come across an app that bundles them all together in one simple to use and beautiful-looking package.
Although slightly pricey in my opinion (US$49 Mac, 19.99 iOS), Capo is a perceptive tool for musicians wanting to learn and develop their skills. With the ease of drag and drop (and of course, practice), Capo takes learning music to a whole new level.
Capo is available now on the Mac and the iOS App Store.