Collision avoidance is one obvious time when steep turns are useful (excuse my droll humour) however it is not the primary reason for the inclusion in the PPL syllabus believe it or not.
I was told that the reason that a steep
descending turn is taught these days (I think it wasn't when I first did my PPL?) is so that you can get down through a hole in the clouds, having stupidly and illegally got yourself stuck on top of them.