When should you try to create a custom loss function?

I was trying to make a custom loss function for a problem I’m trying to solve and I think it was actually just a metric that I needed to display instead of an actual loss function. So now my question is when does it make sense to create a custom loss function and are there examples of custom loss functions in fastai?

Very interested to hear scenarios when other people created custom loss functions and ideally examples of them as well.