http://ux.stackexchange.com/questions/16101/why-do-so-many-websites-have-the-restriction-of-user-names-starting-with-a-letteAn advantage of this restriction is URLs.
If a user's ID is 177896 and their username is H4rold, then you can have 2 URLs to reach this user depending on which information you have at hand:
http://example.com/user/177896http://example.com/user/H4roldThen, on that page, they determine how to look up the user by checking whether the first character is a letter or a digit. Alternatively, they could just force you to include at least one non-numeric character, though that increases the complexity of the check from a character comparison to either a regular expression or a looping multiple-character comparison.
However, not all websites with these restrictions take advantage of this flexibility. My theory is that web developers have seen this restriction so often that they just took it as the norm rather than investigating its purpose... We do that, sometimes.