Without them you couldn’t get off the ground and without knowing which one to use for your particular application you wouldn’t be any better off. Props come in many shapes and sizes. They are also made from many different materials such as abs plastic, wood, and carbon fiber.
Finding out which is right for your application is very important and there are a few simple ways you can make that determination. The first thing to check are the specifications of your motors. The documentation for most motors will generally come with recommended propeller sizes as well as thrust and amperage draw measurements for each different size propeller they recommend. Propellers come marked from the manufacturer using a standardized sizing system.
You will notice on the propeller above it names the manufacturer and also the number 10×4.7. The first number will always be the length of your propeller in inches. The second number signifies that this propeller in a perfect world would give you 4.7 inches of forward travel for every rotation. Typically a number of 4.7 or below is known as a “slow fly” propeller and is generaly used with motors which have a lower kv rating and more torque. Propellers with a pitch larger than 4.7 are typically for faster higher kv motors which have less torque.
Multi-rotor aircraft add another layer of complexity to propellers in that they use both clock wise rotating and counter clockwise rotating propellers. In the case of a quad copter they have two of both. This is a necessity to maintaining stable flight and attitude. On a quad copter the torque of the clock wise rotating and the counter clockwise rotating motors acts to cancel out any tendency the aircraft might have to spin either left or right though no control input had been given. This allows the quad copter to achieve yaw movement by alternating the opposing motors RPM. Propellers are usually designated with a CW or CCW marking.
Much more information on propellers and motors can be found online. In the resource section of the blog you can find some great places to look.