What is Fetch > Automatically in iPhone in Settings > accounts and passwords?
Fetch is for use where Push doesn't work - for instance on POP3 accounts, rather than IMAP.

It simply goes to look if you have any email ready to download, every 5, 10 mins... whatever you set the interval to be.

iCloud mail, for instance can Push. If there is mail for you, it will literally push it at your phone, no checking required.