Like Jim said, instead of a cam with an 'average' profile, you can vary the exact timing of each valve opening to allow the air/fuel charge to enter and then the exhaust exit at a more optimal level throughout the RPM band (i.e. less lift and shorter duration makes for a clean burn at idle and good torque off the line, where a higher lift, more duration, and maybe more overlap means greater high-RPM power.)

Volvo's implementation of VVT is mostly for improved emissions, not greater power. I think the NA engines use it on the intake side only, and the turbos use it on the exhaust side only.