What is the practical purpose of backing up the system databases?

I know that system databases hold logins, security info, jobs, etc. Assuming I have backup of all system and user databases, and the SQL server crashes, then I will have to re-install the SQL server which will have its own system databases. It does not make sense to restore the system db backups that pertain to the crashed server onto the new server (I don't think this is even allowed). So, what is the practical purpose of backing up the system databases?