Adding a desaturated clouds layer above a scene will apply a dappled light effect, which can help liven up a scene, make it more natural and bring it all together. (Using Composite Photography to Create a Fantasy World by Karen Alsop).
Textures are best applied over a blurred background with the main subject sharp, and usually masked out of the texture. (2lilowls).
Textures are more versatile if they are applied in monochrome, as you can then change the colour in Photoshop.
Textures can be added to Ps Libraries, which makes them easy to find and use.
Overlays & Textures to Buy or Download
Brooke Shaden has a good video on using textures and many free textures to download.
Jerry Jones (**) - Wonderful free art textures and overlays