With backwards sorting you know that, unless there is a cycle, you can always scroll up from a call site to find the definition or down from a definition to see where it is used. With forwards sorting you can scroll down to find a definition, unless the function was imported, or used as a decorator somewhere, or called by something that was used as a decorator, or used in some other way that I haven't thought of.

My personal experience is that this predictability is hugely useful. It almost entirely obviates the need for jump-to-definition within a module, and gives modules a very obvious shape and structure.