The Hubble Space Telescope, though, has sensitive optics that could be contaminated by thruster exhaust, so it used magnetic torquers that interact with the Earth's magnetic field for its desat maneuvers. There are advantages and disadvantages to both spin stabilization and 3-axis stabilization. If thrusters are used, optical observations such as imaging must designed knowing that the spacecraft is always slowly rocking back and forth, and not always exactly predictably. Reaction wheels provide a much steadier spacecraft from which to make observations, but they add mass to the spacecraft, and require frequent momentum desaturation maneuvers. No matter what choices have been made, spin or 3-axis stabilization, thrusters or reaction wheels, the task of attitude and articulation control falls to an AACS computer running highly evolved, sophisticated software.
Some spacecraft have components that require articulation.
Voyager and Galileo, for example, have scan platforms for pointing optical instruments at their targets largely independently of spacecraft orientation. Many spacecraft, such as Mars orbiters, have solar panels which must track the sun so they can provide electrical power to the spacecraft. Knowing where to point a solar panel or scan platform, that is how to articulate it, requires knowledge of the spacecraft's attitude. Since AACS keeps track of the spacecraft's attitude, the sun's location, and Earth's location, it can compute the proper direction to point the appendages. It logically falls to one subsystem, then, to manage both attitude and articulation.