Quick question for clarification:
Just as an visual effect? So that it snows/rains in the scene? Or do you mean adding assets (plants) in a specific season? ('both' is not an aswer ;))
I was thinking more the former, although it wouldn't harm to have some season specific assets. For example in Twinmotion, they have 4 of each tree for each different season (bare winter, autumn colours, spring new growth and full bloom summer). But yes, mostly I was thinking the ability to have snow on the ground, or rain with ground 'wetness' showing.