You are not allowed to use cheap keyboards for gaming, The gaming community will stop you! (If that sounded ridiculous, its because it is).
You dont really need expensive keyboards, but many people like them, due to various reasons:
-Features like backlight, stop/play music, etc can be sometimes helpful if you listen to music while playing but you dont want to alt-tab to change a song or pause the music (can be quite good in streaming).
-Mechanical keyboards can be good in some cases, but to be fair everyone is different and mechanical keyboards will only become very important in tournament level competition, if ever.
-Size matters, and your hands and finger size also matter so some people just want a keyboard that fits their needs.
James Mason, I dont know about lol DOTA or LoL (I played only cs go and Id never say that fast pressing was important there), but if there is one game that for me required speed is starcraft broodwar and starcraft 2.
In those games, I can feel the difference from a PS/2 connector and a USB one, and in case its not something you have had some experience with, usually is a difference of 4-25 ms that actually is a problem for me, as keys pressed on the keyboard and mice are recorded in incorrect order by the PC (usually at around 500-700 actions per minute speeds its something rather common).
However, only PS/2 fixes this, no USB keyboard can so far, same goes for mice, but I doubt more than 500 people on the planet run into this problem, so Id assume its not something that bothers most ppl.