i understand the reasoning, but i (mho) don't find that a compelling reason that these things should be readily available to the public.If you had, you likely wouldn't be asking the question.
When training, especially, or simply shooting for fun your standard 30-round mag empties quite quickly. When competing, whether for money or fun, in a timed event where the score is simply a matter of the number of targets hit, for example, having to change mags half as often can make a huge difference in your score. In short, there are a plethora of legitimate reasons your typical rifle owner might have for wanting higher capacity magazines.
As for the bump stock, while the reasons are fewer probably the most common is simply the adrenaline rush you get when throwing that much lead that quickly down range. While it technically does not make a weapon fully automatic, as there is still one trigger pull per bullet, it is a much cheaper, easier, safer way to get the "feel" of firing an automatic weapon without the expense of actually buying an automatic weapon or illegally modifying a semiautomatic weapon to become full auto. However, bump stocks make the weapon much less accurate, so you're sacrificing accuracy for a higher rate of fire.
competitions could very easily just require everyone to use the same capacity mag without losing anything.