The first thing we need to think about is what our render array depends on. Are we rendering some entity data? Are we using some configuration values? Or anything that might be changed elsewhere impacting what we have to render? If the answer is yes, we need to use cache tags. If we don't use them, our render array gets cached as it is, and if the underlying data changes, we end up showing our users stale content or data.
To look at this an other way, imagine a simple Article node. This content can be shown on its main detail page, in a listing of article teasers or even a listing of article titles (and many other places potentially). And since there is no way of knowing where it will be used, it is the responsibility of the render ...
