Why are there alternating dark and light green patches on a football field?

Answer 1

It's purely an aesthetic thing... exactly the same as having stripes on a well-tended lawn, I've seen some non-league football pitches that don't have it.

It's certainly nothing to do with measuring as some have stripes, some have checks and some have circles, or even more elaborate patterns, though you could argue it's easier for players to visualise the pitch from above if it has some sort of pattern on it as opposed to being plain green.