>>32538293Because every Pokemon, when defeated, will give at least one Effort Value point to an individual stat to the Pokemon that knocked it out.
Grinding hoards lets you get 5 EVs for one battle as opposed to just 1, making training a faster process. For example: Lillipup would give 1 Attack EV when defeated, but a whole hoard would give out 5 Attack EVs.
EV training is typically done when you know you have a Pokemon with the right nature, IVs, and ability first because that will maximize its potential. I suggest you look up guides or videos that individually explain Natures, IVs, and Ability breeding mechanics before EV training.
