Not saying ad spend on branding is a very optimised area in general - but the prices of billboards / tv ads etc are sky high and have a much lower degree of targeting. There is an assumption that their prospective and current customers will take note of the fact that the celeb is using their particular product and this will influence a purchase or reduce churn to a rival brand. It may be an effective channel for them.

Bonkers when you take a step back. But logical given the context.