We shouldn't need International Talk Like A Pirate Day either. Because, who the Hell doesn't enjoy talking like a pirate on any occasion possible?
But it's fun when everyone gets in on Pirate Talk together. Especially those who wouldn't normally think of talking like a pirate on that day.
It also brings Pirate talkers together who might not know there are other Pirate Talkers in the vicinity.
"Ahoy Bob!"
"Shiver me Timbers Frank! You speak Pirate too?!"
"Aye Matey, ever since I was a Lad"
And, probably most importantly, when large groups of people are all talking like Pirates on the same day, it raises awareness of said Pirate Talkers. And when seeing Pirates en masse, people are more likely to notice the Pirate Talkers on other days and in other aspects of their life.