Gordo beat me to the punch so I'm just going to repeat.
1) Close Quarters - If you have had to push away, or if you are trying to hold a badguy off, your other hand may be fully occupied.
2) Stealth - Visualize "Star Wars"...Would Han have gotten Greedo in the cantina if he'd had to rack a noisy slide? (yes I know, fictional movie, but my point still stands that stealth and surprise are force multipliers).
http://www.youtube.com/watch?v=e1YbFnkZwZk