Using an Opacity Mask is one of the best ways to add bitmap textures to your vector designs in Adobe Illustrator.
What makes bitmap textures so great? In general, bitmap textures hold much more detail than vector textures.
Vector textures can have a ton of vector points, slowing down your computer and making your files difficult to use.
Using Opacity Masks with bitmap textures are also a great way to set up your designs for screen printing, because whatever is black in your texture will show through as background color. When you output your screen separations, the transparent areas will be there!
Below are links to the texture collections mentioned in this video: