The primary use case is for making sure kids and elderly loved ones are brushing their teeth and keeping the brush charged. Also supposedly senses if all teeth are being properly cleaned, although based on reviews that sounds like a stretch. But yeah, the aggregated data is probably marketable somehow - I didn't check their privacy policies.

Electric toothbrushes are a thing. It's understandable that the vendors would like a rationale to track usage. Not beyond the bounds of possibility that in ten years it may be impossible to buy an electric toothbrush that doesn't track usage and try to phone home to report it.