There'll come a time in your GIMP career where the brushes that it provides for you simply won't cut it and you'll have to create your own. Fortunately, this is much easier than you might think.
Most other sources try and overcomplicate the process of making a brush, but they really needn't. It's even simpler if all you wish to do is use a brush that someone else has already created!
Like most things with GIMP, it's enjoyable to create and play around with the results - the brushes won't go away so you might find a use for them in the future.