PvP rating is the one thing in WoW you cannot farm with patience, and it is where a boost is worth the most. Solo Shuffle rating, arena carries, the Conquest cap so your gear keeps up, the seasonal Gladiator mount and the elite weapon and heraldry rewards are all here.
Rating work is played by current-season gladiators on the region you choose. Coaching is listed too, if what you actually want is to hold the rating yourself next season.
Rating, rewards or coaching?
Buy rating when you need a bracket cutoff by a deadline. Buy the Conquest cap when your gear is holding you back instead of your play. Buy coaching when you keep hitting the same rating wall — it is the only one of the three that changes your results permanently.