I am happier with a round knife, because it cuts curves better and easier than my box cutter, it cuts straight lines through thick leather much, much easier, and it holds an edge longer. These, and the push cut (rather than the pull), makes this blade safer to use, in my opinion.
This last point goes to your post, Windrider. A sharp blade is safer than a dull blade. When working with a dull blade, or a blade that dulls quickly, frustration can set in, and the idea to just pull through that last cut can overcome common sense. When that last cut involves a pull or tug toward you, the danger appears to increase.
It's easy to get the basic use of a round knife, just keep your hand behind the blade. I have cut myself accidentally with a box cutter, but never with a round knife.
(Now, I'm no doctor, nor statistician, as a disclaimer.)
Last -- how much do a pack of razors cost? If you've used 100? Now, consider Ferg's great answer, a 60 year-old knife. How many razors in that time? So, there's a calculation that would be unique to your age and amount of use, but it may well be that a good round knife is an investment that pays off at some time.
So, to answer your question -- why would anyone buy one? Ease of cuts, versatility in thicker leathers, longer time staying sharp,
To answer JLSLeather's question (which was not the original question, but one he read into it): What can a round knife do that a box cutter can't do? Nothing. What can it do more easily, probably more cheaply over time, and probably more safely? Most everything.