Take a look at this example schematic:
The chip in question is the PIC18F4550 and as you can see power is provided on both sides (with a 100nF cap to smooth noise I guess). Is this strictly required or could you put power into one side and just ignore the other? I know that I have just put power in one side and everything still seems to work fine but I feel uneasy with doing that. I feel that they would not waste pins that could be doing something useful by having an extra pair of power in/out pins.
For what purpose does this PIC MCU (and others like it) have multiple power pins?
Answer
Each Vdd and Vss pin (and AVdd and AVss, if present) must be connected, and each pair should be decoupled. Multiple supply pins with their own decoupling ensure that different parts of the chip receive a clean low-impedance supply, which wouldn't be the case if the supply was distributed inside the chip itself.
For decoupling to be effective, the connections to the pins should be short with wide tracks to minimise inductance. The supply track should go to the capacitor and then to the pin, just putting the capacitor on stubs with the supply and ground going to the pins first isn't a good idea.
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