What is the primary purpose of the ISS today?

It's a microgravity orbiting laboratory, in which scientists can run experiments that require microgravity or placement above the earth's atmosphere. Here's a list of ISS experiments performed as of 1/20/16 taken from NASA - Experiments by Hardware: Biology and Biotechnology. Animal Biology - Invertebrates. read more

The main purpose of the International Space Station is to provide an international laboratory for experiments within the space environment. Which, despite all our technological advances, is nearly impossible to replicate here on Earth. Because for many of those experiments, the key parameter is—you guessed it—weightlessness. read more

The astronauts (who are test pilot trained) are responsible for steering and orbital realignment tasks. Also for handling emergencies that might arise during the course of the mission. The mission specialists who are scientists or trained engineers do the experiments which is the main purpose of having ISS in orbit. read more