The wrench is probably the most basic of all tools. There are several varieties including open end, box end, combination (having one open end and one box end), flare nut, striking wrenches, etc. But the primary job of these wrenches is to fit onto a fastener and allow us to tighten or loosen it. In order to do that, the tool must fit the fastener properly. It must offer enough leverage for us to loosen or tighten, and it must fit the hand well. If you examine common fasteners, you will find that most have 6 sides. On the rare occasion, you may find one with 12 sides. Generally, if the fastener has 6 sides, you will get the best fit, and be able to use the most leverage if your wrench has 6 sides. This is important on rusty or stubborn fittings that require some real leverage to break loose. Some 12 point wrenches have a reputation for rounding off the heads of fasteners because they fit more poorly or handle less torque than their 6 point counterparts.
Some years ago, tool manufacturers looked into this problem of 12 point wrenches rounding off fasteners, and began to offer wrenches to address the problem. If you imagine a 6 point nut, with a 12 point wrench on it, you can see that it is imperative that the fit be snug. If the fit is loose, when the wrench is turned, the corners of the nut will be bearing all the load of the torque. On small fasteners, this causes the steel in the fastener to yield to the tougher wrench, and the corners round off. Smart tool manufacturers realized that to address the problem, they needed to shift the load from the corners to the flats of the fastener. To achieve this, they formed the tools so that small hollows are left at each corner. Since there is no part of the tool that comes into contact with the corners, rounding off is practically eliminated. Different manufacturers call this innovation different things, but the concept is the same. To see this in action, view the demo at the Kobalt Tools site here.
So now that we know that 6 point is usually more preferable to 12 point, what kind of wrenches do we buy? Well, open end wrenches typically offer torque to 2 sides of a fastener. Box end wrenches offer torque to all 6 sides. Flare nut wrenches torque on 5 sides. So obviously the box end is the strongest. However, not all fasteners can be gotten to with a box end, as they may have an obstruction or a hose, or wire on them. In that case either the flare or the open end will be needed. Since combination wrenches offer a box end and an open end, most people purchase those first. Don't be surprised to find that premium toolmakers only offer 12 point box ends on their combination wrenches. Their tools are of sufficient quality that this is usually not an issue.