Intonia was designed first and foremost to analyze the intonation of stringed instruments like violins, violas, and cellos. You can play fast or slow. You can study or critique recordings of other players.
As a practical matter, I've found that it doesn't work well to watch the display while I'm playing, unless I play very slowly. There's enough of a time lag that making quick corrections just doesn't work. If I want to tune up a single long note, I can use visual feedback from Intonia to make corrections. But if I'm playing a passage, it's best to play it at tempo, then stop and look at how I did, then go back and see if I can play it better.
Intonia makes it easy and fun to record yourself while you play, then listen to yourself. The visualization of pitch makes it easy to pick out particular passages.
The visualization of pitch makes a lot of common errors obvious. It doesn't take a trained ear to spot sliding into a note. Shifts, vibrato, and articulation are all evident in the trace.
Intonia also works as a transcription tool. I can visually identify a passage I want to transcribe, then play it enough times to be able to write it down. I can even look at the display to identify what note is being played.
And setting aside all music considerations, Intonia is a pretty good general purpose recorder. Audio can be saved in the standard .WAV format, and read in from any source, including tracks ripped from a CD.
You can edit audio files with "Copy", "Cut" and "Paste". You can even copy from one file and paste into a different file.