When should I export (backup)/import a .pst file?

You want to move email from one email account to another email account.

Using Outlook on your desktop, you can add both accounts to Outlook. Wait a while for all of your email to show up (if you have a lot, this can be a long time). Then, use Outlook to export the email from your Yahoo account to a .pst file. Finally, import the .pst file to your Office 365 account.
You're moving from PC to PC: You're moving your email from one computer with Outlook on it to another computer with Outlook on it.

For example, let's say your old computer has Outlook 2007 and you just got a new computer with Outlook 2016. Export your email data from Outlook 2007 (described in this article) and import it to Outlook 2016.

You're moving from PC to Mac: For example, when you're moving your email from Outlook on a PC and importing to Outlook 2016 on a Mac.

You want periodic backups: When you want to periodically backup all of your email, contacts, and calendar to a safe place. After you create the .pst file, copy it to a safe location, such as a USB flash drive, another hard drive, or cloud storage such as OneDrive or Dropbox.
There's no automated way periodically backup everything (old and new). Auto-archive only backs up old items, and it moves them rather than making a copy.

If you only want to backup your contacts, see Export contacts from Outlook. It explains how to copy your contacts to a .csv file.

Regards
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