If you're specifically talking about Rockstead, they have the hole there as their trademark fro their knives, thus why it is always on every knife they make. I believe that it was originally used in construction of the knives when they were sharpening the blades by hand, but now I believe they use a different method to hold the knife, and the hole is still there because it has become synonymous with Rocktead.
Same can be said of some other knife makers/manufacturers. There is largely no need to have a hole in a blde unless it is for opening, or used as a drain in the fuller like Bob W mentioned, but most knives also don't really need a hole in the fuller anyway.
Todd Begg has also become famous for having holes in his blades, described as a "fluted" blade. He uses different sizes of holes, which gives a very good look aesthetically, but really isn't going to serve any major purpose if we're honest about it. No one can deny it makes one mean looking blade though
This one is mine. The machining in the blade will give you a great deal of respect for how precise the work is on the knife, and being able to do something like this on the blade is also a kind of sign of talent or skill as well. It's much easier to see than the other things that make this knife amazing to the average person.