Not sure of the level you're working at, but I'll give you a simple answer.
There are two main benefits of parallel circuits: you get independent control of the components (like in a car, you can switch on the fan without the headlights coming on.)
I agree with Mark C's answer. But wish to offer support for the usefulness of series circuits.
Series connections have uses. I will use Mark's example of wiring in a car. Consider the switch on the fan, the fan's switch must be in series with fan in order to be able to interrupt the power to the fan when turning it off.
Consider an LED indicator light. The current thru the LED must be controlled to protect the LED from excess current. A resistor in series with the LED is the typical way to accomplish that.
So I would say that parallel circuits are more effective in some situations and that series circuits are more effective in other situations.
I hope this helps,