A "football field" is not a unit of measurement, so why does anyone ever use it as such?

I ask because that's exactly what they do in this video (at the 0:17 mark). It's bizarre. Why not use metric units, which is what the whole world is familiar with anyway?

I have no idea how large a football field is, so that bit of information is totally useless to me.

A "football field" is not a unit of measurement, so why does anyone ever use it as such?
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