Very occasionally do I think and image should be B&W whilst taking an image, but would always shoot colour RAW regardless. The B&W Mixer option in Adobe Camera Raw gives a lot of control over the underlying colours of the B&W image, to alter the contrast of areas of the image depending on what the colour was. Much more options than just an orange or red filter with B&W film, which for most was a global effect the user had little to no control of post capture.
Choosing B&W ex post facto, depends on the image.
Sometimes an image just jumps out as being B&W, sometimes being mono can simplify the image, sometimes there is a dominant colour which is distracting from the subject. Or something else.
I also still have the free Nik Collection in Photoshop, and Silver Efex Pro2 plugin has a lot of presets which change a lot of settings other than the underlying colours, which can give very dramatic results. I rarely use them though, mainly because I would like to take more control myself.