Essential

So many times, you Google a WordPress problem that you want to solve. Sometimes, there is a plugin for that. Many times, there is a solution on StackExchange, or StackOverflow, or some similar site, that tells you a function only. With this one plugin, suddenly all of those solutions are as easy as copy-paste. I do not need to back up my functions.php or do any weird hand-stands. I can just add my own function, tailor it a little, and be done.

I once broke my site briefly by adding bad code here. That is hard to do, as the plugin is pretty good at spotting problems before you save. The support forum showed me how to fix it. This plugin is now on my list of essential and basic plugins for all WP sites I build.