Based on my experience as a developer and user I can't understand how PayPal got so big and ubiquitous. I had the worst experience ever in trying to set up our recurring billing system with them, wasting several days of work, to the point that I simply gave up, switched to Stripe, and got everything up and running, in production, in hours. Even as a consumer I had a terrible experience when trying to send money abroa…
Outside of the US, credit cards were far from ubiquitous (and they still aren't). PayPal was the one system that could handle international transfers reasonably.
Edit: Also, speed and payments to individuals. Back when eBay was an auction house for used goods sold by individuals, not a marketplace for commercial sellers, it was the one way to get someone money that was instant. No idea how that worked in the US, but in Germany the alternative was bank transfers which took 2-3 days back then.