There are mods in which docking rings have been completely removed and you just dock with the planet, so it is possible.
However, I would imagine a docking ring could have some kind of energy or field related thing that enables your ship to survive atmospheric entry, either that or it is simply for traffic and guidance reasons. Also, docking rings mount defenses on them, and make sure people can only enter at one point, as opposed to the obvious security and traffic issues of ships being able to land anywhere (e.g. a xeno could easily land on manhattan if it wasnt done by ring, as only a section of the planets "front" has any security or traffic control)
