In commercial photography, one of the main jobs of a professional is ensuring that the client gets the "look" that they want. That "look" isn't just what the model is wearing or doing in an advertisement. The "look" is part of the branding for that client, product, or business. For example, a punk rock band would prefer a grungy "look". To create a captivating image with the desired "look" stylistic photography choices such as grain level and contrast are made during post processing.
Programs such as Illustrator and Adobe Photoshop have allowed for the creative potential of photographers to be unleashed. It is now easier than ever to get the look desired by the client or use the tools to create totally new looks.
Textures are just one tool that can be used to ensure that images used for marketing by businesses have the same look in accordance with the branding, giving a website or brochure using multiple images a sense of symmetry to the viewer.
An antique store could have images of all sorts of whacky items to use for marketing. By creating a subtle retro dust and grain texture to apply to all of the images, continuity between a image of an old western gun to an antique furniture set remains in tact. It is pleasing to the eye, and therefore pleasing to consumers.
To see more about the potential that textures can offer, visit 77 Essential Photoshop Techniques for creating photo-realistic Textures. It is only the tip of the iceberg!