What is the significances if
According to Tim Berners-Lee it "seemed like a good idea at the time":
Sir Tim Berners-Lee, the creator of the World Wide Web, has confessed that the // in a web address were actually "unnecessary".
He told the Times newspaper that he could easily have designed URLs not to have the forward slashes.
"There you go, it seemed like a good idea at the time," he said.
He admitted that when he devised the web, almost 20 years ago, he had no idea that the forward slashes in every web address would cause "so much hassle".
So no special reason, it seems.