As a Roblox developer, it is currently impossible to create or modify meshes at runtime
This would allow for things like terrain using marching cubes, water with waves, dynamic destruction of existing meshes, capes, clothing, efficiently shattering glass, procedurally generated trees/animals, ropes, and probably a lot more
For terrain and water, yes smooth terrain exists but developers get very little control (ability to adjust tint color is all we have) over how its looks and is interacted with, which ends up being a deal breaker for any game that wants to have a unique art style
Example of Unity’s api for modifying meshes