All aspects of a website are important to deliver a successful user experience, even the small parts – like buttons! The interface the designer creates to deliver a portion of this experience is influenced by various constraints — from web standards to the company’s brand.
Consistently using a certain style helps you to create a node of activation. Using these nodes in congruence will lead you to develop an increased brand image with familiar customers. Larger sites have been doing this for awhile now. These elements wax and wane with the popular design styles of that year, but are consistently anchored by a company’s branding guidelines.