If normal speedruns are like playing speed chess, tool-assisted speedruns are like finding the theoretically-optimal solutions to chess problems (https://en.wikipedia.org/wiki/Chess_problem). Both of them require intense devotion to the game, though the first emphasizes physical ability and execution while the second emphasizes exhaustive thinking.

You could consider bicycling tool assisted running. It's just a different take on competing.

Some people also use TAS's to find, reproduce and understand glitches for use in normal speedrun.

People use them as another tool for finding glitches and exploring the possibility space of the game's systems. They're basically using debugging tools and in some cases even looking at memory allocations etc.

As for full length tool assisted runs, I just find them to be a fascinating new lens to examine these games under. It's cool (depending on who you ask...) to see what the game looks like when another computer program is piloting it perfectly towards the goal of 'finish as fast as possible'

Just for fun, or to have a "theoretical optimum" to compare human speedruns to.

Because NASCAR is fun, even if you are going at super-human speeds. And there is skill in maneuvering a car at super-human speeds.

TAS players play the game frame-by-frame and seek theoretical perfection.

It started out for pure entertainment. Now, they sometimes work with realtime speedrunners to optimize their strategies.

If you're just watching it's arguably more fun to watch, since it theoretically represents absolutely perfect play.