Backgrounds pull a lot of the weight in any kind of graphic presentation. Anything on your sites, flyers, or Power Point presentations will be better off with a great background. They are the most effective and transformational design element you can use.
Your background sets the tone for the subject of your design and the accompanying design elements. An introductory pamphlet will have a different background design than the packaging on a luxury product. Likewise, the background on your website can have a huge effect on how your products are perceived.
There is also a well-understood psychology to colours, which are vibrantly displayed in background images. They can be the primal, deciding factor that leads to a new customer, or at least to a new member of your email list. The point is that your background images should be among your first considerations, not your last.