Well for start, you can create 10 different characters on a single account and have 2 of them online at the same time. You don't really need a second account. Aslo if you do something dumb and force us to ban you well, banning one account instead of 184989232 is much less time consuming.
As for account sharing with others, well that only prevents people from giving each other their account details and allowing them to run yet again multiple accounts (explanation above).