Scientific experiments require subjects - which are infrequently humans for many (obvious) reasons.
Although closely related, non-human primates are expensive and have serious ethical questions. Other, smaller mammals are used more regularly, but are limited as they are expensive to house and its hard to use a large number of them at the same time. In experimentation, more "n" or data points is almost always better.
Because of this, researchers look to a host of alternatives. One of the most popular is the roundworm, Caenorhabditis elegans (C. elegans).
It may seem strange that many important, scientific questions are being answered using a worm that is the size of a comma on this page, but it is indeed, an incredibly powerful experimental system...