Most branch lines, industrial tracks, and yards have no signals on the prototype ... and placing visible signals on all of them would be cluttered, and very unprototypical.
Invisible signals can be placed on 90% of your route, to control traffic on these branch lines yard tracks an industrial tracks.
Invisible turnout levers can also be on airplane runways and aerial flyways.
Invisible track speedboards are also handy, where a visible one would be objectionable looking.