Hazards (and in particular stealth rock) are not so much about chip or breaking sashes/sturdy, but rather about being able to pressure and add cost to your opponent's actions. That rocks do type scaling damage is of special importance -- you are effectively limiting opposing counterplay (in a predictable manner) by restricting the switching of certain checks. E.g. my opponent needs Moltres to stop my Scizor. I can rotate my pokes to force moltres in or otherwise get my wincon. Without hazards, the game would just be endless regenrator + unaware battles.I'm not sure where to ask this (it's about competitive play in general, not just OU), but is it necessary (if one is to do well) to have hazards on every team? If so, why? Otherwise, in what case(s) are they not necessary? I never did question it, but stealth rock has been such a big part of competitive play that I just assumed it to be necessary.
Stealth rocks themselves are not necessary -- they are just the best hazard because they always have *some* effect and do OP damage to particular type combos. You can leverage the same pressure on grounded pokemon with spikes or stally stuff with Tspikes.