Why do we need to manipulate the trainer ID? What do it change for speedruns, like Catch'em all or NSC speedrun?

It's for the Cooltrainer glitch, the way i think it works is you need it to be the right when you mess with game memory, it does what you want it too. thats probably not totally it but im thats my understanding at least

In NSC, during the party swaps at the end you swap the bytes which store your trainer ID to map connections, causing you to warp to the HoF by stepping right. In Catch 'em all, during an early underflow menu, you swap one of the bytes corresponding to your trainer ID with a slot in your inventory, allowing you to quickly obtain the item 9f. If Trainer ID manip was not done, these 2 things would give you a random item, or warp you to a random map, which is obviously less useful.