What is the Automatic Forced Subtitle Stream?

Hi, so I don't know what is the "Automatuc Forced Subtitle Stream" used for? My BD's have PGS subs so it seems to me the Forced stream is useless. I would appreciate a hint about it.. Thanks!

A subtitle track can contain a mix of regular subtitles and forced subtitles. This option creates a second track that only contains the forced parts of the original track.

For example you have a BD with English audio and English subtitles. Some parts of the movie have foreign language. If you are only interested in the subtitles of the foreign parts, then you could use the extra subtitle track that was created by that option.