[Higgsboson]

Regarding the gravity issue, all you have to do is spin the sphere. This will get you a band of living area around the middle or the sphere orthogonal to the axis or rotation. The rest is still pretty unlivable, which is why a ringworld is a more efficient choice.

Regarding need, I've already answered this question. I will add something I felt was already obvious: With such a huge amount of living room, you could support a massive population in relative comfort (think trillions; "We need breathing room!)while only having to defend one system.