A tourniquet is a must-have piece for any first aid kit. This BCB tourniquet works the same way as do similar products of better-known manufacturers.
For your information: This tourniquet does not appear on the CoTCCC list of recommended first aid devices. If you only go for CoTCCC stuff, take a look at TacMed SOF Tourniquet and CAT.
Massive bleeding is not unheard of in civilian life. Everyday traffic is one risk, or something can happen at work. If you fantasize about saving the day, carry a tourniquet!
The modern battlefield and mass shootings have shown that the risk of having to amputate a limb because a tourniquet was applied is not really an issue, while blood loss is among the most common reason for the loss of life. This has changed the approach to tourniquets and instead of the last option to treat limbic hemorrhage, they are now pretty much the first thing you do. There's not an IFAK on the market today which doesn't include a CAT.
For more info, here's a clear article with dos and don'ts along with other verified info.