Adds slower accelerating and breaking Trains that in return are able to hit higher top-speeds. This is very useful for trains travelling vast distances.
Mods introducing new content into the game.
Adds technology to research entity ghosts at the beginning of the game. Very useful, if you wanna know right from the get-go what the biters broke, or when playing with Bot-Start or something similar.
Providing the player with new tools or adjusting the game interface, without fundamentally changing gameplay.