Arrrrrrrrr!
There are hundreds of possible reasons, but for the sake of brevity, only a few will be mentioned here.
The simplest and most likely reason is that the letter R is the 18th letter of the alphabet. This number, 18, has always managed to cause a great deal of pleasure for me personally. (Just a moment, 18! 18! 18! Aaaaaah.)Why should you say "Arrrrrrrr!"? Well, the best reason is, as mentioned above, it's fun. Another good reason is because you happen to be a pirate (like myself, "Arrrrrr!") This is simply good protocol for pirates. It is really one of the unwritten rules of piracy; if you don't say "Arrrrrr!" at least once a day, your shipmates are likely to have you walk the plank for land-lubberish tendancies.
A third good reason is if you need to relieve excess aggression, for as we all know, R is a very aggressive letter. (As a matter of fact, in some societies, even saying the letter R in public will get you in a very dangerous situation.) Despite this risk, the letter R should be used more often and better (there's nothing worse than a badly said "Arrrrrrrr!")This concludes my writing on the letter R. I would just like to say before I go, "Arrrrrrrrrrrrrrrrrrrrrr!"