One word: Charizard
In all seriousness, if you scroll down the Top 30 in usage stats, you'll see that rocks hit 5 'mons for at least 25% per switch-in. This, combined with the 12.5% done on neutral hits, can turn a 2HKO into an OHKO. And since Pokémon on BSS teams are usually EV'd specifically to take on a particular threat from full HP, SR damage can often serve to invalidate your opponent's "counter" to something like a Mega Salamence for example.
On a similar note of ruining counters/techs, SR also has the added benefit of breaking sashes on threats like Breloom, Pheromosa, Volcarona, and sometimes Serperior/Mimikyu/Primarina/Blaziken. Not picking up a necessary KO due to an unexpected Focus Sash can cost you the game, so getting up early rocks can definitely save you from nasty surprises later on.
Finally, SR + phaze strategies are popular on offensive teams with 'mons like Hippowdon as it enables Yawn/Whirlwind spam to weaken the opponent's team and allow a set-up sweeper like Zard X or Mence to clean up late game.
