Those units wouldn't exist within the same class if they weren't related to each other. By grouping them into their classes visually, we can take advantage of that relationship to make our diagrams more easily.
Usually, it saves us trouble later on. Things that are related to each other at one level are often part of the same thing at a higher level.
In testing, as in chemistry, it's important to change only one thing at a time. If we pull together more than two things in a single step, we've changed more than one thing, and so we can lose track of where any problems we find came from.
Pop quiz - writing integration tests
The ones in the smallest circles, especially if they don't have any lines pointing ...
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