As a Roblox developer, it is currently impossible to apply moisture to a block.
Overview
I like to use rain in my showcases a lot. Whether my builds take place in a café in Seattle, or in the middle of a crime scene in an apartment building. Rain, in my opinion, is an important piece in building, and it’s honestly weird to me that we have tools to implement fog, but not moisture?
Neon District by InfiniteEffect is a great example of what moisture could be used for.
Showcases like this could use additional texturing to their advantage, emphasizing the rainy conditions, adding a polish to the game.
Before you argue that this can easily be resolved by using Reflectance, take a look at this.
I placed a transparent, reflective, layer of glass over this part, and the result didn’t come out so well.
Conclusion
I believe rain is a core element to realism and detail. Moisture can be used in multiple settings, in multiple ways. Whether it’s condensation on a coffee cup, or the rain collecting in the street. Moisture texture is just another way to power imagination.