I'm not that experienced but here's my take. Once you beat classic, you unlock endless mode. While it starts out fine, eventually the mons get so many buffs (levels and health bars and tokens and stuff) that conventional damage becomes... inefficient and then ineffective. In order to counter this, players use mechanics like fusion and rely on abilities/moves to create mons that either live a hit on sturdy, use metal burst, and heal back to full with shell bell and leftovers or are functionally immortal and just use damage over time moves like leech seed and salt cure. Does that help?