Not only do they provide an environment or background that you can use in your designs, a skilled designer can use them to set the overall feel and mood of the composition as well. If you have a decent camera, you can mostly create the textures you want using a photo of the desired object and some Photoshop magic. Here are some sites you can look up to find free Photoshop textures in case you are feeling lazy or don’t have the appropriate resources to create your own textures.
The sites are in no particular order. [CG Textures] - The worlds largest free texture site. Image * After - currently 27037 free textures and images available. Royalty Free Textures and Background Images - texturevault.net. Free Textures from TextureKing.