That's not the only reason people get credit cards. Credit cards are a primary source of building credit. You won't be able to get a good rate on a car loan, mortage, (private) student loans, etc. without a credit history. Having those installment loans will also help build your credit, but you won't get those loans in the first place without good credit. Also, banks aren't the only ones who look at credit - so do prospective landlords, cell phone companies, future employers, etc.
Like someone else said, the key is to use it responsibly.