Well what about those entertainment corporations that have 10 bands under them, have a high turnover rate, and their top party bands have to stay on top of each new top billboard tune every week? It isn't that these musicians can't play or don't play on stage or that they don't do their homework, because they do, but if you're a serious entertainment company that does their own lighting and sound for every gig for every group, is known for and hired for productions, not "gigs", and known for sounding "perfect" then that reputation has to be upheld. One of the companies I'm speaking of has an inhouse study that they pay guys to track parts for shows in because first off, when you get hired for a NYE gig, outdoors, it's 28 degrees outside, no ones instruments will stay in tune and the guitar player is stuck in traffic coming from another gig but you have 1,500 people in the audience and the city is funding the whole thing, what are you going to do? Play out of tune for an hour and a half with no guitar part? Absolutely not! You start the show with the tracks that were recorded for the show by the musicians playing the show. There are too many factors with a high production value party band that works all of the time, in and out of town, to fully rely 100% on the musicians 100% of the time. People get sick, singers overdo their vocals one night and can't sing the next day, etc. On other hand tracks can be a disaster. For example, cruise ship gig, Broadway show tunes, string parts are tracked, the live horn section comes in and they all sound they're 20 cents flat against the tracked string parts but the show must go on and for some reason no one is stopping tracks. Total disaster. There is no substitution for live music, but you also maybe want a substitution to save your ass every once in awhile IF the need arises.