During the many decades of space exploration and manned missions to the ISS one of the main jobs of astronauts, in addition to safely navigating the stars, has been to conduct various forms of research. The microgravity of space lends itself to all kinds of scientific experimentation and one that's being investigated now is how to use the microgravity environment to grow 3D human tissue. While many cells are being cultured in petri dishes in labs on earth, the shape of these cells is somewhat flat and they don't often grow into the proper 3D shapes that are needed. On the ISS, microgravity is the norm and developing tissue in that environment is likely a much better option that trying to create, maintain and work with a simulated gravity.
In the unique gravity of the space station, rather than being somewhat flat, like a pancake, tissue and cells can be grown in shapes that are round or cylindrical and more like actual tissue and organs. The lower levels of gravity on the ISS allow for the cells to grow and form into shapes that more like what grows in the body, and thus are better suited to treatments for diseases like cancer.