We shall assume for the moment that they are not using traditional acupuncture. There is a style of traditional treatment where needles are inserted and removed almost immediately, but this is because the system is based on an understanding and use of subtle energies. It is unlikely that a chiropractor would be doing this.
It is much more likely that they are using one of the western medical systems, either trigger point acupuncture, segmental acupuncture or straightforward neurophysiological techniques. The former is more likely and is based on an idea that knots form in muscular tissue which can impair the body's freedom of movement. Using needles into these trigger points can release the muscle, and this is often done by osteopaths and chiropractors to enable them to encourage changes in structure.
We are always slightly ambivalent about other healthcare professionals using a small amount of acupuncture. In principle we accept that it will happen, but we have tried for years to ensure that practitioners who take on even small elements of the system are fully aware of all of the health and safety implications and also of proper hygienic practice and clinical waste disposal requirements. These are the same whether someone uses one needle or a million needles every year.
It is always worthwhile asking your practitioner what they are doing. All responsible practitioners are only too happy to explain to people what they are hoping to achieve.