When should I draw the use-case diagram of the existing system? if ever?

Which steps are required to find the use-cases of a software system (automated system)?

Is analyzing the existing non-automated system through a use-case diagram a prerequisite or we can directly start from the use cases of the intended software?

If not, is there any other diagram which can help us in drawing the use-case diagram of the new system? I know some analysis of the existing system is required but I would like to know is there some standard diagrams for such analysis?