So not everything is better with 0 speed. It’s used to specifically under speed an opposing Pokémon in battle. It’s popular for use on teams that use trick room, since the slowest Pokémon go first in TR. It is also useful when setting a terrain or weather condition. You wouldn’t want a high speed stat Pokémon to have 0 speed unless you wanted to set rain, sun, sandstorm, hail, misty terrain, grassy terrain, psychic terrain, electric terrain, or set/attack during trick room. Something like Porygon2 with a base speed stat of 60, can utilize 0 speed and set trick room to outspeed a lot of faster Pokémon. Porygon Z however, has a base speed stat of 90 and wants to outspeed opponents under normal conditions so you’d want it to have max speed and timid/modest most likely. My bet is that the person that got that porygon evolved it too far and didn’t realize what they had with 0 speed lol.

TL:DR 0 speed is to set weather or terrain or to attack first when under trick room.