Why use triangulation for flat terrain?

I saw many places in wired mode, and they all used triangles. I get it if you use it for different heights, but why do people use so many triangles for flat areas in their area? If there is a large flat area, would it be wise to create one large square or at least one large triangle (as many as possible), and not use so many small ones?

So my question is, is there any reason to do this (possibly for textures)? I know that tessellation does something like this, but still leaves too many triangles from my point of view.