Rims usually have small holes inside to vent the inner chambers of the rim extrusion.
All rims need them, but rims with welded seams need to have more and larger vent holes because otherwise the welding process will cause pressure to build up in the closed chambers and "spit" the molten metal. The spoke and valve holes are not sufficient venting because rims are rolled and welded before the spoke holes or valve hole are drilled. Plus, the spoke and rim holes don't always go through chambers at the edge of the rim. Often you see vent holes at the edge for this reason. Bike frame tubes always need some kind of vent hole for the same reason (welding and later draining).
I just built up a Rhyno Lite XL rim last night that had 8 different vent holes! I always make sure the holes get covered with tape, but some of these were at the corner where a regular rim strip wouldn't cover them. So perhaps it is OK to put the tube directly over them? I would welcome input from other mechanics about whether the inner tube can always be placed directly on these holes. My road bike, which I run at 120 psi, was built up without the vent holes covered. Because I was fighting mysterious flat tires, I used epoxy to smooth over the rather large vent holes (no room for tape in that case).
Even if the rims don't have a welded seam, they may still need to vent the hollow extrusion to keep water or pressure equalized.