Craftland.org

4 seasons in the greycliff. During the summer, all the grass turns into somewhat bright (shade of grass in a desert). Fall the trees leaves change colors and grass gets darker. Winter grass is darkest and it snows alot everywhere (every biome). In spring the grass is its brightest shade of green and birds spawn more often.

I totally agree with this

YES !
also should in the summer the Wheat grow faster and in fall give more droops.

Is there seasons in vanilla? And what does it do there?

No seasons in vanilla currently.