Maintenance, tour buses, employees (ect..). It's a business and like any business, it cost money to make money.
It costs money to enter the park, unless you are a resident, local, senior, veteran, military, government employee, park employee, contracted to work at park. The park fees pay for the park and employees.
Probably because they need to maintain it
On five days in 2020, all National Park Service sites that charge an entrance fee will offer free admission to everyone. - that includes Grand Canyon. The next free entrance day is on November 11, 2020, Veteran's Day. https://www.nps.gov/planyourvisit/fee-free-parks.htm
Because they need to pay employees, maintain roads, trails, vault toliets, Provide educational programs about the park.
Because it is a National Park
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