Breloom is quite an interesting Pokemon, being one of the few viable users of a Sleep inducing move in OU. Combine that with the fact Breloom gets access to Swords Dance means Breloom is guaranteed one Swords Dance after it puts something to sleep. Having priority in your arsenal is also a good addition to, because against a Sleeping target and with priority, you can set up easily while not having to be particularly fast to sweep either.
I think the Fighting Gem set deserves some attention too. It's great at nabbing that extra KO you wouldn't have before hand, and also good at bluffing a Choice Item which can lead your opponent to switch into something only for you to switch up moves. At +2, you get notable KOs on a lot of stuff, such as Max HP Rotom-W, Choice Scarf / Specs Politoed and Genesect, which is important because all of these have the potential to stop a Breloom sweep.