Why does USB type-C use multiple voltage sources?
Because the power delivered through the connector is (within reason) limited by the current.  Power is current * voltage so more power can be transferred through the connector by increasing the voltage.

The duplicate set of VBus pins allow twice the current (almost, connector heating is still an issue).

On the flip side, having a single 5V power rail on the older USB connectors (and on USB-C unless you negotiate for a higher voltage) means you can power 5V devices directly or use a 3.3V LDO for moderate power applications without generating a bunch of heat.

Part of the desire to allow higher power levels is to allow the USB-C connector to be the DC input for thin ultra-book and tablet devices as well as fast charging phone devices.