Well its just one demographic first.
There are a large portion of renters who pay more than 1/2 their income.
But why is it such an important value? I'm not sure personally but I can say for the most part many people don't qualify to rent a given location in the first place unless their income to rent price is equal or more than 1/3 their income.
So another way to put it, about half of all renter now don't technically qualify since to rent their current location given they will likely need to pay more than 1/3 their income.
