Scientists often require keeping a stock of biological collections because it is not possible for a scientist to every time go out and get a sample. Scientists may require comparing several of his biological subjects that might not always be found in the area, so it is important to keep a collection. It depends on the type and size of the organism in question. Certain organisms like butterfly may not be available in the area where the lab is located. So the scientist has to make prior arrangement and keep a stock of the organism.