Here's the part in question (2955):
While Technic parts typically have a recess to allow the ends of pins to be flush with the surface, this part (and perhaps others) carry this recess all the way to the edge of the part in a U shape.
I looked through instructions from a few sets to see if I could figure out if there was any utility to this, but I wasn't able to find anything. Here's an example of the part used in the Mobile Crane (8460):
Is the some purpose to this design, or is this just a quirk of this part?