You are correct that there is a connection to ensuring fill levels, but incorrect on it not being intended to provide structural integrity. It is both.

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You can also view the original patent here: patents.google.com/patent/WO1999022994A1/en

Notable excerpt from patent:

When the horizontal ribs are not provided completely around the container, the face panels may be provided with indentions of preferably a circular configuration. The size and depth of the indentations may be varied to control fill level of a given volume of contents in the container in addition to further stabilizing the sidewalls.