Try a little test : google the questions words in English.
On top of the list, you will find the usual suspects, what, who, why and how.
One will be quite less used than the others, whose.

However, the whose question has been the most important, controversial of all.
It has been the cause of so many wars, murders, conflicts and tears.
It's the essence of what makes humans different.

For anything, you can always ask : "whose is it?".
When this is a person, the answer has been agreed by blood and sanctuarized by a famous text : a person is self's.

But for anything but a person, the question remains: "whose is it?'.
Knowing this is the most essential, because we've been (quite) agreeing that the one whose it is can do what he wants with it and especially gives it to someone else.
To this question are only two answers: it is either a person's or persons'.

When it's a person's, it can change faster than when it's persons'.
Indeed, a person has just to agree with himself to do something to it.
When it's persons', they have to agree on what to do with it.

The most used rule for one that's persons' today is the one of the most.
What to do with it is what the most wants.
Most often, this is implemented by voting.
People are most often voting in two cases : for what to do with a state, where a vote is a person and for what to do with a company, where a vote is a share from the company.

The biggest difference between the two are :
For a company, only people that have shares can vote. These people BOUGHT these shares so took a risk to care about the company. They can SELL these shares when they don't care anymore.
For a state, only people that are citizens for this state can vote, so:
People living in the country where a state is can not vote.
People "investing" a lot in the state are as important as the others.
Citizens can not escape from the state to give away both obligations and rights got from the state.

Whatever the case, persons' is most often slower than a person.

So, while I am hating the system for states, I do think that anytime we can have one that is a person's, we should make it.