The reason, of course, is that each brush offers something different. A paddle brush for example is great for smoothing, detangling and boosting shine, while a round brush is great for creating curls. Depending on the effect you want, the tools of the trade are important in helping you achieve it.
The same can be said of the brushes used in Photoshop. These are painting tools that apply colour with strokes by adding a shaped mark on a layer. They will infuse your work with depth and flow which can be further enhanced by the variety of brushes in your design arsenal.
There are many resources for free brushes to add to the presets already included in Photoshop. With spring in the air here's a sampling of what can be found in that theme alone:
Spring Photoshop Brushes
20+ Spring and Floral Brushes for Photoshop
Spring Sketches Brushes
Spring Flowers Photoshop Brushes