Backup your website.
But your web host does that for you! Yes but what if the backup failed?
Another copy of it on your computer is easy, so do it. It also gives you a way to make changes to your site and see how it works, by changing the copy on your computer. You can make it behave as it does on the webserver. See if everything works ok, then, if it does, you Publish your website from your computer to the webserver.

Most of the time you can make minor changes directly on the webserver with no problems. But at that point the copies that exist online and on your computer are different. Maybe it is unimportant and you just don’t care. OK, fine. But if you have changed the name of a document or if you have changed a link, look out! It may create a problem later and you will not remember doing it and you won’t know what went wrong. I’ve had it happen many times and most of the time, I am the problem.

Having two, different copies of the same subject is not good. So just do this, whenever you are making changes to your website, online, at the webhost or local, on your computer. When you finish publish the website to the other location. That makes both of them the same. Now there is less chance of a problem, less chance of error. See the article complete with screen shots here
http://dollarware.us/websitebackup.htm