You probably dont need it

Growl is a weird kind of app. It doesnt do anything by itself, it enables other apps to send notifications to the user. That means that, in order to be worth downloading, it needs to have a sizeable library of applications that use it. But fewer and fewer apps seem to be using it because the developers cannot assume that users will have Growl installed on their machines. I would recommend purchasing Growl only if one of the following conditions apply:

1) You are a software developer and want to include optional Growl support in your application, or
2) You are using software that specifically requires Growl to enable important features.

QUICK HISTORY LESSON:
When Growl was first released, there was no Notification Center on the Mac. Growl filled that gap, and other app developers were happy to build in support, or at least enable Growl plugins, because it was the best and most widely used notification handler out there. It was also free, so it was a small thing for an app developer to tell their customers "You can enable useful notification features by installing Growl for free!"

Then two things changed. First (not necessarily chronologically), Apple built notifications into the operating system. Some people still preferred Growl because it had features that Apples built-in notifications lacked, but a majority of Mac users had never used Growl to begin with. Apples implementation was certainly "good enough" for most users and developers to display simple alerts.

Second, Growl version 2 was released as a paid upgrade. Users were free to continue using Growl 1 for as long as it continued to work, but future support and updated features required money. Customers now had to decide: are the extra features that Growl has and Apple notifications do not worth the price? As more developers included native notification support and fewer required Growl, that price became harder and harder to justify.

I personally purchased Growl because I used another application that required it. I dont remember which application that was, and I dont use it any more in any case.

TL;DR: If youre not sure if you need Growl, you probably dont. If you do need Growl, you wont have to wonder.

Wonkey The Monkey about Growl