If you’re a web developer, interested in security, or just trying to troubleshoot a server problem, then it can sometimes be useful to see exactly what’s powering a website.
The extension adds multiple icons to your Firefox address bar, each one representing a common web component (PHP, jQuery etc). There’s also a separate Wappalyzer icon, and clicking this displays the full list, sometimes with their version numbers.
Clicking the name of any package displays some statistics about it: the total number of sites where it’s been detected, the sites where it’s most often been found, how its use has varied over time.
If you’re thinking that means Wappalyzer must send its detection results back to the developer, then you’re right. This is done anonymously, though, so doesn’t constitute any significant privacy risk, and if you’re still unhappy then it’s easy to disable (click the icon, select Settings, clear “Anonymously send reports…”).