I wouldn't consider any Pokemon really pointless. Some have a purpose in specific gimmicks, some add to lore or worldbuilding, others are good in battle, and some are just fun or interesting. No Pokemon really lacks use or reason.
when encountering both metapod and kakuna in the wild, the only move they have is harden, magikarp on the other hand, the only move it has is splash until it evolves into gyarados
Slaking, Regigigas and Archeops, based on ability.
Greninja and Charizard based on designs.
Slaking(wow slaking sucks), Archeops(why gamefreak)and Regigigas(come on gamefreak its the same strength as slaking), based on competitive.