Stocks help you steady your rifle and get a better, more accurate shot. They do this by providing a point of contact to your body, causing less sway and movement. The most accurate shot you’re likely to make probably involves a bipod or stand of some kind, and it definitely involves a stock. So what are some of the features this simple yet necessary part offers? 
     AR stocks have part of the buffer assembly located within them. This is a necessary part of the AR that causes it to reload the chamber while allowing it to have less felt recoil. Not all buffer assemblies are compatible with every build so keep that in mind when choosing a stock. When you’re considering a stock though, the buffer system is likely one of the last things you are thinking about, just keep it in mind. 
     Collapsible stocks allow you to adjust the stock, in turn adjusting the pull rate. This can help you fire more comfortably and therefore accurately. Depending on your needs you might just need to adjust it once and for all, getting it right where you’re most comfortable. If so it can still be important to have the ability to adjust it, even if you’ll only be doing it once. Or you might need to adjust it often. For example, if you are shooting with a backpack or other gear, depending on the weather or the situation. In these cases, it is good to have a stock that gives you options such as these. More options are generally better, so it might be worth it to pay a little more to get something that is more versatile and which works better for you. 
     With less gun, you might need less support. Pistol braces are technically not stocks, but work great, especially if you are running with an AR pistol. Pistol braces have less material than your traditional rifle brace and less support. A pistol brace can be used to steady your pistol AR and shoot it with just one arm instead of both. If you do it like that it is best to strap it onto your arm with the provided strap. There may be no reason why you can’t use both arms to shoot an AR Pistol, even with the minimal stock, just put it up against your arm, aim and fire like normal. It should be noted however that doing just that has at times been illegal, turning your AR pistol into an AR short barrel rifle, just by the way you use the brace. 
     Of course, you might want a sling. Not all stocks have a place for you to place your sling, but most do. Many stocks have multiple places for slings, important so you can have it exactly as you want it. If you have a two-point or three-point sling then you will almost certainly be putting that on your stock. 
     A cheek rest is important for the same reason that a stock is: accuracy. It is important to become one with your weapon if you want to optimally use it. Get your cheek on it when aiming, become one with your AR through your cheek. A cheek rest can really up your game, allowing you to have a usual position for your head and eyes when aiming. It also can make your shooting position more comfortable, which is key to marksmanship. Just keep your eyes parallel to the ground, so bring that stock up to your head, then rest that cheek on it. After practicing shooting from the same position, over and over again, you can really get a feel for your rifle and where exactly that round is going to go when you fire it. Cheek rests are for the serious shooter, and not surprisingly you are going to have to pay a bit more for being serious. Still, there are cheek rests out there that attach to stocks, so if you are buying a stock that has a cheek rest as part of it, that can equal a great deal. I suppose you could say you get similar results without a cheek rest, but almost all of the best shooters use them, maybe they know something. The Sopmod stock is popular due to its use in the military, and though it doesn’t have an adjustable cheek rest, it does still have a cheek rest. 
     Stocks are there as a tool, for stability and to provide better aim. You can always decide not to have a stock, with just a buffer tube. But seriously, a stock of some kind has gotta be worth it. If for no other reason than to have a convenient and comfortable place to put your cheek.