I played along with almost an hour of backing tracks on YouTube this morning. What a great way to start a day of practice. Started out with an extremely long track, a minor blues in A.
Damn thing goes on for over 21 minutes! Now, if you aren't a musician, you might wonder what the use of a 21 minute backing track might be.
First, I play rhythm guitar at every major position on the fretboard. So, in this case, I'd start with the A minor chord at the open position, then move to the same chord at the 5th fret, then on to the 7th fret, and finally, at the octave. I've cycled now through all the dominant chord voicings in this key.
I'm primarily a rhythm guitarist in the Old Dawgz. Getting fully into rhythm and being in tune is not easy. Guitars really never tune perfectly. They must be played in tune, and intonation changes in every chord voicing.
If you move from one backing track to another, you will hear (once you get good at it) that even tuning machines are tuned at slightly different pitches. Some machines really are 440. Others might be 435 or 445. Moving from backing track to backing track is an exercise in tuning on the fly, which is a very good (and difficult) thing to do.
One of the really distinctive things about the 60s and 70s music that everybody loves is the constantly changing tonal center. Tuning machines were not common. Musicians just tuned to the piano or the bass. So, in that 60s and 70s music, going from one tune to the next meant re-tuning.
Performing conditions can, and often are, quite extreme. High heat and humidity are common at outdoor gigs. Your guitar goes out of tune quickly and you have to either play out of tune or tune to the band as it plays.
Once I feel like I'm really deep in the rhythm pocket of these super long backing tracks, I'll move on to playing lead against the track. Playing lead is never really as extemporaneous as we pretend. Great lead players have a huge library of lines in their heads that they've heard or played themselves. Those lines get stitched together in a variety of ways.
So, as you can see, I find a use for the entire 21 minutes!
For a performing musician, practice isn't just about technique (although that is very important) or about learning tunes. Practice is also about anticipating every possible outcome when performing, and preparing for each of those outcomes.