Donations help pay for the running of the site, let you express your thanks for the tools on the site and help keep me motivated to develop new features.
Your payment is processed by Stripe (for credit/debit cards) or PayPal. To use PayPal you must first have a PayPal account. Both payment processors use secure encryption to keep your details safe: none of your payment information goes through my server. If you donate I can access your email address and may use it to send you a one-off personal email of thanks.