I understand this to be a desireable!
In case of unwanted attention whilst out doing ya bit, you can demonstrate your permission. Other function is to protect your pitch.
Some permissions may want to limit or control your activities, which can be formalised. For night shooting you need written permission if you are not the land owner.
I personnally do not have written permission, I shoot on land owned by good long term friends. I tend to leave the car in a place where they will see it so as they know you are about. Sometimes I call them in advance to OK it, as they may have beast out etc.
If I was to gain permissions outside of friendships, I would like to have a written agreement in place to protect yourself.