So you have just set up your self-hosted WordPress website. One thing I would always advise is to make sure you know how to access your site files via FTP (and/or cPanel's File Manager, which serves a very similar purpose). But why, when you already have an admin login to your WordPress dashboard? Here are five reasons:
Sometimes, when you update WordPress itself or a theme or a plugin (or more likely, a whole bunch of them at once), the site will get stuck in maintenance mode. And when that happens, there's nothing you can do to sort it out via the WordPress dashboard as you won't be able to access it.
However it's usually a very easy fix via FTP. Simply go to your root folder and delete the '.maintenance' file. More about getting stuck in maintenance mode
Sometimes you might want to take a copy of a plugin you are using. Perhaps the plugin is no longer available in the WordPress plugin repository, and you want to use it elsewhere. Perhaps you are about to upgrade a plugin, but want the ability to re-install the previous version if need be.
This is also a simple procedure via FTP. Go to the following directory: yoursite.com/wp-content/plugins and find the folder relating to your plugin. Copy it to your desktop (many FTP clients will let you drag and drop it). That plugin is now backed up!
It's also straightforward to remove (delete) a plugin from your site via FTP. Go to the same directory and delete the folder relating to the plugin in question. This can be necessary if WordPress prevents you from deleting it via the dashboard for whatever reason.
And if you need to disable all of your plugins in one go, perhaps during troubleshooting: rename the 'plugins' folder to something else ('plugins-temp', for example). When you need to re-enable them, just rename the folder back again. This is much quicker than disabling them all via the WordPress dashboard.
If, like me, you are in inveterate tinkerer, it is likely that at some point you will want to make changes to a WordPress theme's template files. Before WordPress 4.9, it was very easy to make changes - especially php changes - via the 'Theme Editor' (found in the dashboard under 'Appearance') that would break the entire site and prevent access to the dashboard at the same time.
WordPress 4.9 made this harder by introducing error checking. However this error checking conflicts with some plugins and triggers the following warning:
Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.
Whether you have killed your site or or are trying to do so but are unable to save your changes (!), FTP will come to your aid. It is very easy to take a copy of the theme template file in question, edit it locally using a text editor such as Notepad++, then save the new version to the server.
It is also worth noting that WordPress' Theme Editor still does not have versioning or change history (unlike the post and page editor), so FTP offers a mechanism for taking backups of particular files prior to major changes.
I hope this has shown you why FTP access to your WordPress files is so useful. It's not something you only want to start thinking about when you have broken the site and locked yourself out of the WordPress dashboard!
So if you have a self-hosted WordPress site, do make sure you know how to use FTP and what your credentials are. And File Manager within cPanel makes a good alternative if you are unable or unwilling to install an FTP client.