Why do Oregon farms plant red clover every spring? (a.wholelottanothing.org)

To provide free nitrogen fertilizer pulled from the air.

That is a really exciting answer for the red clover, but I have more questions. Why are the farms in oregon doing this? Can farms in any geographic area plant red clover? Why are the flowers red but the leaves green? Do I have a similar red clover growing in my yard, that is just red clover leaves with small purple flowers?

*crimson clover (by the looks of it)