Why do people raise nest boxes

I would like to add some best boxes to my portable coop that the girls can use if they go broody to raise the little ones. My current boxes are on the second level and are roll away. I need the room upstairs for more roosts and thinking of doing away with the roll away feature. It is nice but I have two hens that lay their eggs outside the coop and thought it would encourage them to lay inside if other eggs where there.

I guess my question is why do people elevate the nest boxes? I was thinking maybe 3"-5" off the ground would work.

Plus I would like my little girl to be able to get the eggs out of the boxes