system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What do electronic artists (producers and/or DJ's) actually do live?
gpt
There's a couple things that a good EDM DJ can do during a live show, but for every good DJ, there are a dozen others that just do nothing.

1) Tweak the lows/mids/highs to get the sound right. This is especially necessary if you're going from venue to venue and not using your own equipment. Getting it to sound "right" often varies track to track, and so continued adjustment is fairly common.

2) Multi-track mixing. A good studio example would be Daft Punk's  Alive: 2007 album, where they mixed two of their own tracks together to create new tracks. This can be done live, but usually on a much lesser scale, which is more of point 3.

3) Adding samples. A lot of EDM is nearly wordless and very repetitive. This isn't a bad thing, but a track can get old if you don't spice it up a bit. A DJ might throw in pre-recorded add-ons, such as the good ol' "Drop the bass!" or a "Let's go!" DJs that make their own music might even have a pre-programmed sample pad that lets them get different drum loops going or change what synth is doing the melody.

For Tiesto specifically: he's very very good at reading the crowd. He knows that the crowd wants to see him jumping up and down and doing things, even if he's just playing pretend. If he doesn't look into it, the crowd won't be into it, and that's part of why he's so successful.

As for why they don't recreate their music on the spot, it mostly has to do with how the music is made. Most EDM producers nowadays work in software that would be... challenging to reproduce live. There are often multiple different synth layers, a bassline, drums, sfx, and vocals at the same time. Reproducing that with one person doing everything live isn't possible.