When to add another server?

Hello Community

    When using Windows Server and the hard disk starts getting full,
you can add another hard disk or when you don't have enough memory
you can add more memory.

    So when you are running out of hard disk space or memory
when do you have to add another server instead of adding more hard disks
or when do you have to add another server instead of adding more memory?

    Thank you
    Shabeaut