Minecraft it is mostly the names for RP servers, but you also have trolls with multiple accounts to create havoc.
And with Steam it's cause a lot of the times you can only have one game per account, and people want multiple. My roomie says she likes having two accounts so her single player is private and hidden and her multiplayer is open and free for others to comment on.
And with Paypal, it's not always illegal, some people don't want their personal purchases crossing with their work/earnings. Which keeping them separate is more logical and easier for taxes anyways.