Probably to target an ad for the same product/service at someone who was in the same room as a TV ad. About 10 years ago I worked for an ad targeting company and we got ~50% more click-through on a web ad just by showing it shortly after a TV ad aired in that location (just using the geoip timezone and hoping they might've been watching the right channel), if you could do that only for people who've actually been exposed to the TV ad there's the potential for huge uplift there.