Why you may want to stay at POP over Buena Vista Suites (I'm guessing that is where you mean?)
#1 No need to rent a car. No off site resort can match the ease of getting on a Disney bus. Off site buses typically have a set schedule, and don't drop you off at each park. And the bus stop will be out in the parking lot somewhere, not where the Disney buses are.
#2 Disney Magical Express. No need to find a way from the airport to your resort. Or back to the airport when your trip is done.
#3 Extra Magic Hours Especially during busy seasons, they are worth their weight in gold.
#4 Being in the Disney bubble. I love not having to see the real world for the duration of my vacation. I like being in the Disney bubble, where everything is lollipops and sunshine.
#5 No need to pay for parking at the theme parks. If you rent a car and are staying off-site, you have to pay $14 a day for parking.