Because most sellers do not have credit card merchant accounts. Paypal allows you to pay (for free) using your credit card to a seller with a PP account.
Or to your sister to chip in for that baby shower gift for your cousin in Moose Jaw, without sending her a cheque or cash.
Paypal is also an anonomizer (it must be a word, I just used it!) allowing you to send money to a stranger anywhere in the world without exposing your financial information to the internet.
Sellers pay a fee to PP for using the service. Buyers don't.
If you back your PP account with your credit card, the payment is instant.
You can also keep a balance in the PP account or attach PP to your bank account. While both of these are safe, they do not allow the secondary level of Buyer Protection that a credit card attachment allows.
PP accounts backed by a bank account actually issue 'e-cheques' which can take up to 10 days to clear.
Most of that is not relevant to you, but many others will read this is future.
