Another reason for raising nest boxes is to make it convenient for you to gather the eggs. Some people with bad backs don't like to bend over.
If a nest box is on the floor, it is possible the hens can scratch trash from the floor of the coop into the nest. Whether this is a problem depends on how dirty the floor is and how your nest box is constructed.
I had a problem with chickens scratching out the nesting material and fake eggs until I increased the size of the lip on the nest boxes to at least 5". After that, everything that I wanted to stay in the nests stayed in the nests.
I have access holes to get my eggs from outside and never use them. It is easier for me to go in the coop to get the eggs, plus I like to check on things in the coop. I've had a few surprises. A black snake eating eggs. An injured chicken. A possum. Yeah, I like to check.
You can avoid these problems with proper constructon techniques, but those access holes can be ways for water to leak into the coop, for drafts to get in the coop, or weak points for predators to get in the coop. In certain climates, eggs may be more likely to freeze, especially if the nest boxes stick out. That last does not sound like a problem for you, but I'll just mention it.
There can be good reasons to have an external door to the nest box. If you have a child gathering the eggs and you want to protect that child from a rooster, maybe, or you often gather eggs when you are wearing your good clothes and especially shoes, they can be a good idea. If you have a small coop where you cannot walk in, they may be a very good idea.
We are all different with different circumstances. There is seldom one answer that is right for all of us. Nest boxes on the ground, at shoulder height, or somewhere in between will work if your circumstances support that. Good luck!