Having a deep understanding of how something you built functions can provide a sense of ease and confidence.
When you know the details of a system down to the smallest component, you are better equipped to troubleshoot any problems that may arise.
This sense of understanding not only empowers you to feel capable of addressing issues, but it can also bring a sense of calm as you know you have the knowledge and skills to resolve them.
In short, having a thorough understanding of the inner workings of something you have built can bring a sense of ease and confidence that can be both empowering and calming.