Thanks, Lydia!
It kind of depends, but the biggest things for me are colour and texture.
In this shoot for example, not everyone’s outfits matched (either the backgrounds or the other outfits!) so I would use B&W to hide the colour clashing. Other times, it’s good for muting the background and highlighting the foreground (think contrast). This was great when my cousin and aunt were standing in front of some tall grasses; the B&W muted the green grass to grey and focused on the subjects. Another big thing is texture as B&W often pulls certain textures in images. In this shot particularly, I had too much texture competing for my attention. By going B&W, the rock column, path, and my cousin are the focus but the grasses and foliage in the background kind of just fade out. B&W is also a good way to solve skin colour issues. Some of my lighting in this shoot wasn’t ideal, thus making the subjects too red or orange, but turning the shots B&W fixed that better than altering the individual colours did.
Generally, texture shots are my favourite for B&W, but it’s great for colour correction, too! When I do B&W though, I pull the saturation slider all the way left and the vibrance all the way right. I like the effect better than the menu option for B&W in Lightroom since it’s brighter and then I don’t have to do as much exposure compensation.
Hope that helps! 😀