Biologists study the incredibly useful zebrafish to learn about everything from evolution to embryonic development (and sometimes a bit of both). Why use this particular fish? Because it has an amazingly fast gestation period (among other things). In the video below, you will see a zebrafish go from egg to fully formed embryo in less than 24 hours. With such a quick development, biologists are able to study various aspects of evolution and development quickly and efficiently.
Another reason to use the zebrafish is that the gestation period is remarkably specific. Leave a fertilized egg in the lab and 22 hours later you will have a swimming embryo/fish. The embryos are also see-through, making it that much easier to track and classify development. Zebrafish too have a fully sequenced genome on file to aid researchers.
However, there is a downside to using one particular fish so heavily in the biology lab: they are terribly inbred. All of the zebrafish used in all the labs across the world come from nearly the same line of individuals collected nearly half a century ago. Because of this inbreeding, there is little variation between each zebrafish. Because genetic diversity is the stuff that evolution feeds on, it is hard to make a good evolutionary study out of it. But as you saw, being able to grow a fully formed fish in less than a day certainly has its advantages (say you are studying the effects of turning on/off a certain gene and want to quickly see how it affects the individual, for example).
Zebrafish also have the ability to regenerate fins, skin, the heart and the brain (in larval stages). How cool is that?
Much more information on zebrafish here.