As Zerdun said, it allows the passive to charge on more spells at once, but at the same rate as before, more or less.
Currently, you can trigger is 2 times in 8 seconds, which sounds like 1 per 4s, but that's not exactly true.
Using Lux as an example (Seraph on Lux is decent if you build around it, but I digress)
A normal rotation is QWERE or QERE if you don't need shield. Well, the second E doesn't trigger because it doesn't have a mana cost, so we have QWER/QER. With the current setup, if you QER someone, you only get credit for the Q and E, even though you used 3 spells, you used up both of your 2-per-8seconds charges.
With the current PBE changes, you get 3 charges instantly, and then 3 again as soon as your 12s is up. So it's better for quick fire combo mages, like Lux, Cass, or Anivia.
So it is a little bit of a buff, without actually changing the charge rates.