Another great question. Several reasons create the need to have these holes in the bottoms of your specialty corner cabinets. From our perspective (manufacturing), we can only machine one side of a part, and this particular part needs to be machined on two sides if you choose to utilize leg levelers (the leg levelers require a 15mm hole to attach the legs, but we also have to groove the other side of the part for the backs).
To solve the need to machine on both side in the absence of the ability to do so, we choose to through bore the leg leveler holes (so we machine the interior face, but through bore the leg leveler holes so they can be used from the exterior face).
From the users perspective, it is very difficult to reach the back leg(s) of either of these specialty cabinets, and by through boring these holes, you can adjust this hard to reach leg or legs from inside the cabinet (just stick a long Philips head screwdriver down through the hole, and you will find a star shaped bottom of the foot that will allow you to adjust using the screwdriver).
Once you are done leveling your cabinet, you can use an 18mm peel and stick FastCap to cover these holes.