I have a couple of scopes with integral rail. Mine have the older-style rail which is trapezoidal in cross section, with the “leg” members (which join scope to base) secured in place by clamping and crosspins. There are a couple of other, proprietary systems now, with various ways of securing the leg members in place on the rail. Zeiss and Swarovski scopes of current production each have their own individual systems.
I think there are a number of advantages. Among them, you tend to have more latitude for placement fore and aft. With a crosspinned system, or one with teeth such as Swarovski uses, you are not depending on friction to prevent the scope moving under recoil. You don’t have the risk of damaging or adversely affecting the scope’s function by overtightening rings. The rail may also add some stiffness and robustness to the tube too, and of course it allows the designer a free hand in choosing tube diameter.
Disadvantages, if you could call them that, are that you are limited to a fairly small number of suppliers of mounts, and the mounts and scopes tend to be at the expensive end of the scale.
There’s no good reason for the latter though, save that only the expensive makers seem interested. FWIW there was at least one scope line made, a generation ago, which was fitted with integral rail but was not at the premium end of the price scale, and that was the original (Japanese made) Nikko Stirling Silver Crown Mountmaster. I had one of these on a .22 for years, and it gave good service.
