For example: TV show X licenses pop song Y for its initial broadcast. The group that owns the syndication rights didn’t buy the right to use song Y, consequently they have to replace the music with generic Muzak in subsequent broadcasts.
You pay for Netflix, start watching show X, and when it gets to the big dramatic moment with sweeping music (that you remember from when the show originally aired), it’s replaced with elevator music.
Same goes for a lot of movies.