As Mirage said, you have access to summons, who you can manually upgrade through spheres and you have overdrives. Rikku's trio of 9999 + Wakka's Attack Reels = 119988 damage even without any extra stats.

Not to mention it is also possible to almost finish the whole game (only second form of BFA wasn't yet defeated, it's incredibly luck based fight) with these restrictions NSGNSNCNONENNENBB, so No sphere grid, No summons, No customization, No overdrives, No escape, No no encounter, No blitzball. The whole game is pretty much based on Rikku's steal and use though. Still it's fun to play.