If you are pointing about creating more than bitcoin wallet, maybe there are features on other wallet that is present on other wallet especially web wallets. Since buying and selling bitcoin is less hassle way if exchanges will be used, people are choosing exchanges that is fit to them. Also creating other bitcoin wallet is a good way of not storing all bitcoin on one place.

If you are pointing about addresses, well most of the bitcoin users needs privacy that's why they are using multiple addresses in different transactions.

If you are pointing about bitcointalk forum account, obviously for a purpose of signature and bounty spamming.