I won't speak to the Asterix specifically, but I will more broadly address the issue of resupply and how important it is for a navy with any aspiration for going beyond your own coastline.
In the simplest terms possible, the joint support ships that are under construction in Vancouver at the moment are a variation of what is called, generically, “underway replenishment” capability. In essence, it allows a group of ships to operate farther afield because they carry with them, in that group, their own logistic support, which includes fuel, ammunition, potentially food, and other supplies.
As it relates to the numbers and how significant this is, in essence it's a force multiplier. If you have a group of ships of whatever size—two, three, four—and you can keep them at sea longer or you can extend their reach in terms of being able to go farther afield, that's where this joint support ship capability comes into play.