There are many reasons why people buy Apple products. But Macs do not come with an instruction manual, if they come with anything at all. In fact they come with less and less as time has gone by.
While many of us feel macOS is designed so well that it is self documenting, not all users have an easy time.
Mac owners who want a resource to get them over the initial speed bumps or solve some problem will, I hope, find the information below helpful.
And don't forget, you can always ask MacGuru it you can't find and answer below.
If you believe that Murphy was an optimist then the following is for you.
Let me say again that this is not a "requirement" BUT if you have been having problems (there are never any problems with a Mac are there?) or if want to be sure of not loosing anything and being able to restore your old OS then this should give you the best chance.
At a minimum I would have a full backup before any "major" upgrade (i.e., 10.1x to 10.13). But all of you are backing up all the time already using Time Machine, right? If you are running macOS and Time Machine you have macOS Recovery built in. However, macOS Recovery will ERASE your disk before it runs. Time Machine does not back up your entire hard drive but Carbon Copy Cloner does. Accept no substitutes.
So here goes:
NOTE: It is more than likely that the new install will reset some settings and files back to "factory" defaults. This will require you to reconfigure those settings. (i.e., New major versions of macOS ALWAYS overwrite the httpd.conf file. So if you use your Mac as a server you should make a copy of this file before you update.)