The iPhone's stock Camera app is one of the most well-designed apps I've used over the years. It's fast, free, and customizable enough to handle the needs of most people. Apple's done a great job with it, but that doesn't mean that Camera is the best app for everyone. It lacks the kind of manual control that pro users love, and it has a few quirks some may want to avoid, like adding Apple's own post-processing to every picture.
Third-party camera apps are a great way to fill those power user gaps, and even if you don't need those kinds of features, they also offer a different design language that you may like more. Plus, you can still add these camera apps to the lock screen, which makes it even easier to try them out, and see if you'd like to upgrade from Apple's Camera app.