The necessity of having a backup of your account emails is a common one because of various reasons - to make sure important emails can be re-instated in case of accidental data loss, when preparing for migration, etc.
To back up emails with Outlook, go to File > Open & Export > Import/Export:
In the next window select Export to a file and choose the extension you would like to back up the emails in - .csv or .pst.
You should choose the format depending on your needs.
If you need to back up emails to keep reserve copies - any will do.
If you plan on migrating the emails somewhere else later, it is recommended to check the supported format with the other mail provider:
In the next window select the folder you would like to back up, target location and backup name:
Click on Finish to complete the process: