Functional Fluids is a tutorial mod, designed to fill in the gap in (Neo)Forge documentation. Like any good tutorial, it has fun examples:

Cool Lava - Tired of cobble gens that burn your cobble? Want to live in a fire lake, but don't want to burn down your base or guzzle fire resistance potions? Cool lava is the stuff for you - all the properties of real lava, with none of the burn. Supports boats.
Thick Air - A balanced alternative to creatived flight. Invisible by default, thick air can be walked through just like the regular stuff. However, when you start sprinting, (or leave the ground for any reason) you'll start swimming! Safe to breathe; can fall, but doesn't flow sideways. Ideal for invisible paths and airlocks. Most mobs are afraid to touch it, but can be pushed through.
Configurable Fluid - Need a custom fluid for your modpack, but not sure how to add it? The configurable fluid has (most) of its properties controlled by the config file. Push strength up to max for a fast transit system, or change the color with the season for automatic festive decorations. Change the fall damage multiplier to 100 and make compact death pits or disable drowning and build an underwater base.
River of Time - Children flow up the timestream, adults flow down. Timeless entities (players, golems, and inanimates) don't flow at all.
Flood - Grows to fill any container. Perfect for swimming pools, watery megafarms, and artificial lakes. This is dangerous (CAUTION: don't use on top of a mountain!), so there's no default recipe. It can flood your world.

I'll probably make the fluids individually disableable at some point - for now, you can use a datapack or crafttweaker/kubejs to manually remove the recipes.

Now with Wiki! (A fluid API wiki, that is. This is an example mod.)