Whips are the summoner's melee-range weapon — a swing tags whatever it hits so your minions focus that target and deal bonus tag damage. They scale with attack-speed bonuses and never use a minion slot, so every summoner build runs one for direct damage.
Kaleidoscope, dropped by the Empress of Light, has the highest tag damage and best summon scaling. Dark Harvest and Morning Star are strong late-Hardmode picks, and Snapthorn is the go-to early whip.
A whip is a summon-class melee weapon: striking an enemy tags it so your minions focus that target and add bonus tag damage. Whips scale with attack-speed bonuses and don't take a minion slot, so summoners swing them for direct damage on top of their minions.