For those who don't consider Iberia's objective coolness to be a good enough reason, consider the following benefits:
Naturally defensible terrain.
In the middle of the most important waterways: the Mediterranean and Atlantic oceans.
Sparsely garrisoned by collaborationist troops, who may well be turned to our side.
Post-capture, we will be within immediate reach North Africa, France and Italy (with its precious, precious Tiberium).
Sounds like a good central base to me!