Purpose of high side in a half bridge?

Quick question.
What purpose does the high side mosfet of a half bridge serve?

rephrased:

If my load is connected from Vcc to the Drain of the lowside, what does the high side do on the opposite pwm input signal?

tnx

You are using it as if it were a single transistor.

The idea behind a half h-hbridge is that you can switch between + and GND.
Like a SPDT switch.
If you put one on each terminal of a DC motor you can run the motor in both directions.

Lots of info on the web. Google for it...