So that your logo can spread the message about the distinguishing characteristics of your business
Professionalism in the quality of the graphics, the printing and the paper on which your materials are printed.
So that your logo looks equally good on both a business card and on a sign for your business (or a billboard!), and at every size in between. Your business's name should be legible at different logo sizes.
Consistency in use of your logo, tagline, materials. Repetition of similar elements, used in the same or similar ways, helps people to remember who you are and what you do.
Which helps you stand out from the crowd. For example, if everyone in your industry uses a particular symbol (i.e., travel agencies often use globes in their logos), try to use something else
Unity among the different elements in the logo. The logo must fit together as a single unit, and not just appear as a jumble of elements pasted together.