Crowded parks mean ...
... guests can't get done all they want to get done so buy more day tickets therefore trapping them at Disney.
... guests can't get done all they want to get done and don't have days/time for Universal etc keeping all the money at Disney.
... lines are long and time is precious so guests don't leave to eat off site or in room ... they pay the prices even if unreasonable.
... parks are crowded, guests want that break time so they book TS filling those chairs.
... parks are crowded and by natural process you get pushed into shops and likely will spend money.
... when parks are crowded they can convince many the only way to gain advantage is book Disney hotel.
... when parks are crowded they can use that as excuse for less street atmosphere, eliminating parades, selling upcharges.
When parks are not crowded folks need smaller tickets, are getting enough done to leave to lay at pool (non-cash generating) or drive offsite ... don't feel compelled to eat in park, don't feel the pressure to pay for advantages such as Dessert Parties ~ EMM ~ DAH ~ Character Meals etc, add days at other brand parks or attractions, drive offsite to explore shopping options and are more comfortable with off property stays. Crowds and demand mean more money for Disney. Crowds are key to sell everything else besides tickets.