Size is important, but quality is also very significant too.
I suppose its a little bit like saying you can either have ten ingrediants in your kitchen cupboard to last you all year, or you can have three, but you can change them every week.
As to what its meant to achieve.
I think its the legal definition for free range meat hens.
So if you want to call your meat hens free range, the inspector will make sure they have at least that amount of space each.
Its a fairly good amount to be supplemented by free ranging and a moved run every now and again.
If I was doing a mixed run, I double it, at least, and do something about entertainment.