Using custom brushes can be useful for adding unique backgrounds or strokes to a design. But one uncommon way to use them is to make their stroke very large, causing some interesting effects for use as backgrounds or other abstract designs.
As an example, let’s click-and-hold onto the Rectangle tool, which will bring out a side menu which includes the Ellipse tool. Select it and draw out a circle. Hold Shift to make a perfect circle.
On the Control Palette, change the stroke to a different brush.
Choosing this example brush will change the stroke to something like this.
Next, change the size of the brush to a higher number.
The circle doesn’t look like a circle anymore. This causes this interesting look, but let’s make something a little bit better.
Select the Star tool from the same toolbar area.
Change the Stroke of the brush to 100 pt and to this brush shown here.
The effect causes it to not look like a star anymore, but this abstract background.
Ctrl-click (PC: Alt-click) and drag the object to a duplicate, and do this twice to make two. Change the color of the Stroke of the designs, and adjust the Opacity in the Transparency palette.
Finally, place a raster image (File>Place) and apply a Live Trace (for this one, I used the Type preset).
Click Expand, and use the Direct Selection tool to select some parts of it and delete them. Now we have a unique, abstract design which can be used as a background or even a logo.