If you are not familiar with the main series Pokémon games, weather effects in the game were also another part of the battle mechanics.
Weather situations like a sunny day, rain, snow, hail, sandstorm etc. were a major part of the main series games. There were actually some Pokémon moves like Rain Dance and Sunny day designed to bring in such weather conditions. Also, the mighty Tyranitar with it’s Sand Stream had the ability to bring in a sand storm at the time of the battle.
In previous alleged leaks we have seen about time of day, and weather effects being added to the game are cool and all, if there were supposed to be true (which we believe they are), then we should have seen them by now in the game. Unless Niantic is cooking up another strategy to make things similar to the originals.
Having these weather effects provide strategic advantages to specific types of Pokémon and make them stronger in certain use cases. For instance, Pokémon with water types moves deal more damage when it is raining, and Pokémon with fire type moves deal more damage when it is sunny.
Also, there are certain series of Pokémon, like Lotad, which have abilities like Swift Swim, which raises their speed stat, whenever it starts raining.
The original main series games also had moves like Chlorophyll and Solar Beam, the amount of HP recovered depends on the weather condition, if the weather out there is sunny, more HP would be recovered and as for Solar Beam.
But there are some real-life problems too, for example, tropical countries are hot most of the time and it would be raining most of the time in areas with perennial rainfall.