I played in pit orchestras in high school and college, and it's surprising how much can change from show to show. Lots of songs have vamps (sets of four or eight bars you repeat over and over until something happens and it's time to go to the next bit), or holds, or other various other things where the conductor is paying attention to what's happening on stage to cue the musicians. You could probably rig something up in QLab or whatnot to maybe emulate that effect digitally, but it's never going to be "just hit play on the canned track."

People go to Broadway shows to see live performances, and music is a big part of it (in some shows, the biggest).

You may say the audience won't know the difference. But word will get out. Audiences may not be willing to pay premium prices for prerecorded content.

Actually, what is the reason for that? If we had computer-linked humans act a scene, then record all of their interactions, and then have them perform that ten nights in a row as live marionettes replaying their original actions from the computer, is that worse or better?

Does the potential for failure and variation attract the audience? Or just the fact that the medium is much higher fidelity: stage actors are human, three dimensional, and no other medium can record all the subtleties of motion and expression just as well.

It's not the fidelity, it's the human factor.

An concert, a play, or any other live performance is much more than a single night, 2 hour performance. It's countless hours of practice, errors, emotional up & downs during the journey.

The performance one watches is the tip of the iceberg and the underlying depth is what amazes us as humans. The unmistakable performance or real-time and invisible improvisation during the show.

I used to played in a symphony orchestra. Even the slowest of classical pieces is a storm for the orchestra, if not for the spectators.

Because then the people on stage can't react to the audience reacting. That's part of the live experience. Every live show is a bit different because every audience is different.

The reason for that is that most humans like other humans.

Variation is absolutely a huge part of the draw. Broadway fans absolutely see the same show multiple times and compare the different performances/actors.

I agree. I've only seen a couple of broadway shows, and simply seeing there were actual people playing music in the pit enhanced the whole experience (occasionally music was played on stage too in the shows I've seen).

On a side note, these broadway performers and musicians were awesome and incredibly professional and talented. I didn't know what to expect (as a kid, I found these musicals so boring) but it was great.

If it's for theatre performance, I'm guessing subtle timing is important.

Yes, this is a major reason.

Because it's a slippery slope. Before you know it, you'll be listening to material that can't be produced live.

Same reason that people hire a mediocre DJ at a wedding instead of just having a cousin operate an iPod playlist?

And what's that?

Ease and quality of improvisation? Does the musician here improvise? Slight variation in each performance?

Compare Jimi Hendrix's studio recordings, with that of his live performances.

The main song in Phantom of The Opera, in the West End, is/was done to a click track - and the performance is/was noticeably flat because of it, in my opinion.

I'd say it's the human aspect. Wedding and Broadway you go to see people live.

Otherwise why not just fire up YouTube out Spotify

Control.

All of the other answers have good points but are missing the one that matters: the musician’s union requires it. Every Broadway theatre has a minimum number of musicians (depending on the space) if a musical is being performed.