For me as a customer, having PayPal as payment option increases the propability of purchase. Especially with one-time purchases from small vendors. Reasons: No risk of credit card info misuse (by vendor), I only need username and password and I get certainly at least some kind of receipt for accounting.
I’m always surprised as to why people are concerned about card details misuse. You are not responsible for card-not-present fraud (without 3D Secure). You can dispute any fraudulent charge with your bank, and I’d bet good money the bank has actually better customer service in that regard compared to PayPal.
International consumer finance is a mess. Not everybody who shops on the Internet is a US citizen with a credit card account in a first-rate bank. There are other options.