I've noticed that since Rainbow Six and now in Primal. The game can be started and played before the whole game is load in the computer of the customer.
This is a very nice feature of Uplay and Ubisoft games. They are advanced compare to Steam now with this.
And it work well. It did work well in Siege and now it's proven to work well in Primal.
I wonder if this feature could be seen someday on Steam. So for now, Uplay advance in my "estime".