There are several reasons you might want a stock on your gun. The first is feel, because for some players firing with a stock can make a day of shooting both more accurate and more comfortable. The second is look, because a stock gives a paintball gun a classic MilSim feel. Lastly, stocks are essential when using a drop forward.
There are several different types of stock. A fixed stock is what is commonly standard. These stocks simply attach to the butt of the paintball gun and extend it so the gun can be braced against the shoulder.
Extendable stocks allow you to adjust the length of your stock. The folding mechanism is internal to the stock. These are probably the most popular stocks on the market today.
Folding stocks allow you to fold the stock along the side of the gun to get it out of your way for close quarters play.
Sliding stocks slide along two rails on either side of the marker. For combat closer in, sliding stocks slide alongside the marker.