Only to stiffen it making it act as one, maybe for a heavy light fixture.
To prevent fires from spreading, especially at a drop or closed soffit. Or at a curved ceiling for fire stop- installed at the beginning of the flat ceiling part.
Over any and all walls below, for fire stop, as in a basement finishing. Could possibly use draft stopping- check with local codes at B.D.
Blocking at ceiling/wall areas for fire.
Required if ceiling >1000 sq.ft.& ceiling hung from bottom of joists.
Pick one, or what is the application? Be safe, G