The Shuttle IS a part of the ISS whenever it docks. It becomes another large area where the astronauts can move back and forth. But they can't dock it there forever, otherwise, what are they going to come home in? They can't just roll down a window and jump out and fall back to earth...
I think what you might possibly be asking is why they don't leave a space shuttle permanently attached to increase the living area of the ISS. And the answer is, the space shuttle is NASA's Space Transportation System. After Challenger and Columbia were destroyed, NASA has only 3 left, and they are far too expensive to leave them attached to the ISS, they are needed for more NASA missions. Bringing up a docking module that is designed to stay there is a much cheaper way to increase the size of the ISS, and that is precisely what they do.
The Space Shuttle docked with the International Space Station (ISS) during its missions. The shuttle used a docking mechanism called the Orbiter Docking System to connect with the ISS.
If you mean by the roll they played, they were mainly used for going between earth and the ISS, repairing sattelites and telescopes, and doing experiments in space. Now the space shuttle fleet is retired and are in museums.
The most common usage at the moment is resupply for the ISS
Apollo, Shuttle, ISS (and Mir)
During launch the space shuttle speeds up to 17500 MPH. In orbit the space station is going about the same speed. So the shuttle does not have to slow down much. At docking the relative speed of the ISS and shuttle is about 1-2 inchs per second.
The space shuttle currently being used for missions is SpaceX's Crew Dragon spacecraft, which is used for transporting astronauts to and from the International Space Station (ISS) as part of NASA's Commercial Crew Program.
The Space Shuttle Atlantis.
The are reusable space exploration tools, and they are used for satellite repairs and ISS construction.
The Space Shuttle was the first US spacecraft built to accommodate more than 3 astronauts. After the retirement of the Space Shuttle, most cargo is carried to the ISS by unmanned rockets.
So they can find where they are going, like the ISS
Discovery (STS-96) made its first docking with the International Space Station (ISS) on 29 May, 1999. This was also the first shuttle to actually dock with the ISS. The pilot of this mission was Rick Husband, who would later lose his life on-board the shuttle Columbia (STS-107).
The International Space Station (ISS) can be viewed from anywhere on Earth where the night sky is clear and the station is passing overhead. This can vary depending on the station's orbit and your location. Websites and apps like Spot The Station can help you track when the ISS will be visible from your specific area.