Whereas the antennas achieve passive gain by focusing the energy, amplifiers provide active gain by injecting external DC power into the RF cable. This power is sometimes referred to as "phantom voltage" and is carried by the RF cable from a DC injector to an amplifier. There are two types of amplifiers: unidirectional (which only increase the transmitting power) and bidirectional (which improve the receiving sensitivity as well). In addition, both amplifier types come as fixed or variable gain devices. For a network design purpose, fixed power gain amplifiers are recommended for overall stability reasons and because all necessary RF power calculations should be done prior to the network deployment and you should be aware of your network power needs. Traditionally, amplifiers are deployed to compensate for loss due to significant cable length between an antenna and the wireless device. It is unlikely that you will need one in your penetration testing procedure, as it is cheaper and more convenient to use a highly directional antenna. However, if you have additional cash to spare, you might want to purchase a bidirectional amplifier to use in conjunction with the directional antenna for typical power-demanding security experiments such as long-distance connectivity and traffic analysis, or jamming and Layer 1 man-in-the-middle attacks. Unlike the actual network design case, variable gain amplifiers are recommended for testing purposes, security testing included. For example, you might want to tweak the amplifier power to find at which EIRP a Layer 1 man-in-the-middle or DoS attack will succeed.
The main problem with using amplifiers for security evaluation is providing a mobile power source. For this reason, amplifiers are rarely used by casual attackers. However, the use of one by a determined stationary attacker cannot be excluded.