Frankly, in Gen 1 it serves as a sneaky way to empower the player. Most of the trash you fight will give you speed EVs which leads to your pokemon generally attacking first, which leads to easier farming and just a general feeling of player agency since their pokemon always take the lead during an engagement. Whether or not the inclusion of EVs is a good thing is a matter for further debate, but that's one silver lining I see to them in terms of game design that facilitates players getting to enjoy a bit of power fantasy.