It doesn't make sense. After all, anything after the colon is already aprt of the URL, looked up by DNS and matched to an IP address. Where does the colon fit in?
Tim Berners-Lee (or, I should say Sir Tim Berners-Lee), official inventor of the Internet (It wasn't Al Gore, sorry) recently divulged in an interview with the NY Times that the forward slashes are something that he would do away with if he could invent the Internet all over again. He's sorry about the error, and he regrets the amount of trees that died, the amount of finger strain, and the amount of syntax errors induced by the addition of the nearly useless and now ubiquitous forward slashes.
The forward slashes are basically a way of saying "from here on, I will be giving you an address for a file location." Well, duh. That's what the colon after the protocol is telling you. It's obviously redundant. And, considering how many times any web address has ever been typed on a keyboard, that's an insane amount of wasted forward slashes. I don't think anybody's done a study on it yet, but I can give somewhat of a crack at it.
According to Nielsen, Google was getting 5.1 hits per month as of 4/2008. That's 10.2 billion forward slashes just from google alone. Considering that Google had 62% of searches in that month, that means roughly 16.4 billion forward slashes were used on search engines in April of 2008. Now, let's assume we're using UTF-8 encoding (just for kicks and giggles. You're most probably using ISO-8859-1 (Western Languages) but they aren't all that much different). That means there are 8 bits of memory used for every character. That brings our calculation to roughly 131 billion bits of forward slashes used by search engine URLs in April of 2008. That's roughly 15 Gigabytes of forward slashes. Assuming that number stays roughly the same through 2008 (it doesn't, but remember we're just kicking and giggling), that's 180 GB of forward slashes in the year 2008 (1.571 trillion bits). That's just in 2008, and just for search engine URLs. No Facebook or MySpace. That number barely scratches the surface when it comes to forward slash wastefulness. Consider that eastern countries use bigger, more robust (read: memory hogging) character sets, and that every single time someone uses the Internet, two forward slashes are used, and you'll finally start to understand the staggering implications of a simple apology of "Ya, we probably didn't need that." And considering that the syntax is far too ubiquitous to ever be changed, this waste will only proliferate.
So, next time you see those forward slashes adorning your browser's header, think of the trees. Think of your children, and your children's children, who might not have clean air and water due to the sheer waste of energy caused by irresponsible use of the double forward slash. In fact, every time you contribute your forward slashes to the horrid institution of forward slash waste, you kill a puppy 5 generations from now. A puppy dies every time you use the Interwebs.
So, I say "No, we don't forgive you, Sir Tim!" We don't condone the murder of future puppies, and we don't condone people who contribute to that brand of evil.
Just letting you know.