Same reason you have a burglar alarm on your car even though you lock the doors.
The more obstacles you put in a thief's path, the less likely he is to keep working on your phone. If it's Donald Trump;s phone, it's worth a lot of work, but yours? Encryption is a lot harder to break (meaning orders of magnitude more work) than a password, so unless you have the password to your bank account, and over $50,000 in the account, it's not worth trying to break the encryption on your phone. Your password? $2,000 would make it worth doing that.