It doesn’t seem like a perk, until you look at the flip side.Disney has been allowing all guests to enter their parks up to an hour lately for several months.
Guessing this perk is simply disabling access for non-resort guests. It doesn't seem like an added benefit, but assuming they enforce it - it will be a nice add.
By doing this, Disney have removed any rope drop advantage for offsite guests. That is actually quite detrimental for non Disney resort guests who are used to rope dropping, as the lines for the big “E-ticket rides” could instantly be about 60 minutes at the official park opening.