There are two very bad things that can happen to the Zotero data that lives on your machine.
1) You can drop your machine unretrievably in the ocean.
2) Your data can get corrupted or accidentally deleted. I have dozens of Zotero collections with thousands of cites and hundreds of full text PDFs, and my data has never gotten corrupted. If you are using Zotero in the recommended ways, I am pretty sure that data corruption is extremely unlikely at this point. But, you never know. And user errors of various kinds can always lead to accidentally overwriting or deleting data you actually wanted.
Here are the three back-up strategies that protect you against data loss. You should do (1), and also either (2) or (3).
1) Follow Zotero Best Practices by creating an account at Zotero.org and consistently syncing your data. This way you always have a copy of your data in the cloud (not dependent upon your machine). This, however, is not a fool-proof data backup.
Data corruption or loss can affect synced data at Zotero.org as well as the data on your machine. Accidental overwriting or deletion can propagate to your Zotero cloud space very quickly if you use automatic syncing. So you should also make a hard copy back up (regularly, but less often than syncing). You might want to keep 2 or more copies on different storage devices. There are two ways to do this.
2) If you already make complete backups of all the data on your hard drive, then you are all set. When you back up your hard drive, you are also backing up your Zotero files (although if you selectively backup only certain directories, you should make sure that the directories containing your Zotero data are included in your backup -- see the the default Zotero data locations here). If you suffer data corruption, you can use the instructions here to retrieve your Zotero data from your hard drive backup. Obviously, you will lose any Zotero data that you added since your last hard drive backup.
3) You can easily make backups of just the files containing your Zotero data. See the official documentation here. This is unnecessary if you do (2).