Except that's only one end of the vacation. Vacations also have a tail end. In which case, they may very well want to hang out in the lobby before they leave. I'm certain I'd rather hang out in a nice hotel lobby over most airports.
The lobby also serves as a place to meet up. When i travel with extended family, we can't all fit in one hotel room. A lobby or lounge is usually where we opt to gather.
Another time I find myself hanging out in the hotel lobby, including the Poly, is when I'm waiting on a table. WDW has absolutely NO problem making me wait for my seating time, despite making an ADR 180 days out (before COVID-19), and arriving ahead of my ADR time. Very specifically, the last time I ate at 'Ohana, we waited over 45 minutes past our ADR time, which was crazy in 2019. The waiting area by the restaurant was packed, so we walked around and explored the lobby.
In the before times, hanging out in the lobby of GF was one of the highlights of visiting WDW. The live musicians were simply outstanding! Not only did GF have a live orchestra, they also had highly talented live pianists that were simply a joy to hear. Going back approximately 10 years, Poly also used to feature live musicians in the evenings.