The array object as a data structure is defined and created at runtime. You can have multiple instances of these, of different sizes (shapes). It starts out emtpy, and it's contents can change during runtime.
The array editor creates a file that describes the contents of an array. It is fixed, and doesn't change during runtime.
The array editor is a (relatively) new feature of Construct, it didn't exist in Construct 2. Before, you would need to create your array in a third party program/format, import it as a project file, and parse it properly to load it into a runtime array. The array editor gives you a built in way to create these files in a compatible format you don't have to worry about parsing. You can think of it as a built in, lightweight Excel.
Basically arrays as objects can be more flexible than just loading one static set of data (even if you don't use them that way in your particular application). So rather than 1 step "Create array with these specific contents.", its "1. Create an array, 2. Define the contents".