What wood systems need
Sweeping removes creosote, the flammable glaze that causes chimney fires. Alongside it, the damper, smoke chamber, firebox mortar and liner all get checked because heat cycling is what breaks them down.
What gas systems need
Gas appliances vent combustion gases that include carbon monoxide. A blocked vent, a spider-webbed burner orifice or logs sitting in the wrong position all cause incomplete combustion. Soot on the glass or the logs is a warning sign, not decoration.
What both need
Every system needs the top of the chimney intact: a cap to keep rain and animals out, a sound crown, and flashing that is not leaking into the wall.