Each piece of the old M track has a small semi-circular hole cut out of the base of the ballast so you could pass under-ballast cables out into the open and to their final destination without disturbing the horizontal level of the track. Or even run cables which have nothing to do with the particular track section right under it and out the other side.
Of course, if you are making a permanent layout you would just drop the cables through a hole in the baseboard and make the ongoing connections under the board. But if you were making a carpetbahn or a table-top where you couldn't drill holes through the base it was a convenient feature.
C track doesn't have this possibility. So if you are making a temporary set-up how do you bring the cables out without tilting the track at the point where they emerge? Or do you just have to live with this tilt and accept that C-track is designed for layouts where the cable connections are made sub-base?

I agree that this is an oversight on Märklin's part, because C track is perfect for temporary layouts in almost every other way.

I usually create a small incline with some thin plywood pieces and then run my wires under the C track in a gap between the pieces. It is not a very elegant solution, but it works. The dremel modification that Peter suggests is probably a better alternative, which I will have to consider when my layout comes out of storage in a couple of months.

It's funny how opinions differ!

I always thought the semi-circular holes in the side of M-track to be its least attractive feature. On photos of my old layout I would laboriously 'Photoshop' then out of my pictures. For me it was a relief when I changed to C-track to find it was no longer there!

As Mike says, there are pre-marked places on the underside of C-track pieces where one can cut a discrete section out for a cable or two to pass.

Why not just drill holes in baseboard under the track to hide the wires

Sorry guys ,my mistake

I always wondered the same thing - they are not really large enough to pass an adequate power cable through IMO. You would have to machine cut or grind them out to not damage the track and most likely do a larger area than what that internal depression defines. I do think that they may just be a structural element cast into the base.

Considering how Märklin documents their product I predict that we will never actually know.

I will just stick with my elevation kludge until I start to build my permanent layout.

That three meters of .1% grade makes all the difference on a carpet bahn layout anyway.

Yep, I get your point. It brings to mind the anecdotal riposte of Talleyrand to someone who said that the kidnap and murder of the Duc d'Enghien by Napoleon was a crime. 'It was worse than a crime, it was a mistake.'