Imagine you're the driver of a car. You're an expert at driving that kind of car. Someone gets in the back and says, "Please go to 123 Main St.". Another person gets in next to you to tell you how to get there, they are your navigator.
You are the software engineer. You understand the mechanics of actually making the car go. The person next to you is your manager, and the person in the back is the PE.
So what has the PE and your manager done here? Well, what you didn't see is that your manager went to a bunch of meetings before getting in the car to find out that 1st avenue is blocked up and will help you avoid it by navigating you around it. And the PE? The PE had 12 meetings before getting in the car with both internal and external leaders to figure out where the car will go.
Without the PE, you would just be driving wherever you want, and maybe you'd get to a useful place and maybe you wouldn't, but with the PE there, you'll go directly to a useful place. And if you ask them why 123 Main St is the most useful place, they would probably be happy to tell you.
To you it may look like the PE is just sitting in the back, but that's because you don't see all the work they did before they sat in the car, and you haven't asked (or they're bad communicators and haven't told you).