Usage of extensions for protocol conformance in owned objects

[phoneyDev (Stern)]

This is a frequent question. See previous discussions like

What is the "official" ruling on organizing code by splitting large files into Extensions?

The problem: Some class etc. definitions cannot help but grow very large, into thousands of lines. It can feel intimidating, annoying and wearisome to scroll through everything when you just need to focus on a specific subset of functionality. Example: Take a class for scenes in a cross-platform game. It has tons of methods and properties for each category of functionality: setting up the graphics engine, handling physics, managing audio and music, processing touch input for iOS, keyboard input…

Why are extensions used so heavily?

When I look at declarations in Xcode, of Section for example, I see a main declaration and a bunch of extensions. What is the advantage of that over just having them in the main declaration? In a related question, when you want to have a special init, but you want to retain the default implicit init, you have to put the special init in an extension. Why is this better than simply not turning off the default init if you've included one with a different signature? For context, I see extension as…