Music editors do a lot of musical work!
Existing music is cheap. It's easier to throw ten pieces of music at the screen than to write ten. The reason for throwing music at the picture is that the two have an alchemical relationship and you can never REALLY tell how they will work together except by watching them together. Each shapes the pacing & emotional impact of the other. Thus, "throwing a bunch of pieces of music at the screen" is a really good way to learn what works and what doesn't, what makes the scene feel too fast or too slow, what is too little or too much of an accentuation of the emotion or action of the scene, etc.
The goal should never be to copy the temp, but rather to learn what works and why.
I would compare temping to that scene in every detective movie where they sprinkle the chalk dust on the notepad next to the phone in the murdered character's apartment, and by doing that they see what number was written down on the sheet that got torn away.
By throwing music at the scene you get a sense of its contours and pacing and how a potential, original piece of music would best serve the picture.
Although if it's music from the TV show's library or the film's master suite, it could actually end up getting used.
I worked on a kid's tv show and in every episode (the show was fairly formulaic) there was a "What did we learn today?" scene. You know like where the character who hogged their toys learns that sharing is fun. The composer had written about fifteen different variations of music. Some of the pieces felt like pep talks, others felt like they would work as music for a character apologizing for misbehaving, others had a dramatic build to a "lightbulb moment" and so on. Now what was interesting & kind of magical is that in each episode, as I temped in the music, I would always bring in all fifteen themes for the "WDWLT" scene and try them out one by one. And just by trying them out I would find out that some totally flopped against the picture, while others completely transformed it unexpectedly.
You can have two pieces of music that anyone would describe with the same words, e.g. "sad, introspective music" and they would have COMPLETELY different alchemical reactions with the picture.
