In a weird way, this isn't a benefit of staying on-property, but it is an extreme disincentive to stay off-property.
If you're going to stay at a non-Disney hotel or condo or townhouse, you won't be able to beat morning crowds to any of the popular attractions. Flight of Passage, Runaway Railway, Slinky Dog, Smuggler's Run, Seven Dwarfs, Test Track, Soarin', etc. will all already have lengthy lines before you're even allowed in.
Theoretically.