particles in the atf are actually friction materials kinda like your brake pads. this is what makes the gears engage and stay engaged throughout the gear ratios with the help of the line pressure off course. overtime, these particles wear out just like your brake pads and becomes part of the atf. when these friction materials worn out, the friction material just becomes metal to metal and the shiftpoints just rely on the friction dust that is going around on your the atf.
now if you drain your fluid, that is how you damage your tranny. undesired shiftpoint, shifts too early, shifts too late. etc