Accessory scams.

Apart from being illegal, the problem with these devices is that they only boost your outgoing signal. You still will not hear people better, and you will be getting to people who you cannot hear shouting back.
Avoid these devices, a better antenna is always the best option.

There are several reasons why these devices are not as good as they may appear to be. Possible reasons include:
1. The reason that you cannot hear the distant stations is usually that they are no stronger than the noise. When you boost their signal, you also boost the noise - they still cannot be heard over the noise.
2. A CB receiver can only stand a certain amount of signal. Too much signal coming in will cause the receiver to stop working properly, and may even damage it.
3. Higher gain figures just mean that the device is more likely to overload your CB receiver, causing the above problems.
Avoid these devices, a better antenna is always the best option.