I consider photos for bw if the transitions or separations in the photo come from brightness rather than color. For example the picture below just had washed-out sand tones, not adding much imo). I'm trying to get into this mindset when shooting bw film, but it's not easy being used to color as the default
Lastly, low light situations such as concerts in tiny venues often force me to convert to bw so i have to shoot with that in mind, but that's more working within technical limitations than a deliberate choice