It's a way to cheat on Halo -- the host of the match can do wicked things if they have a modded Xbox and modified level maps (like run super-fast, jump super-high, have automatic aiming, change the weapons on a level, etc.). Inducing lag into the game can help the modder get the "host" designation so that they can do these things.
They're not trying to slow the game down, they're trying to cheat.