1) Get your attention.
2) Implant your company name into the minds of viewers and to be remembered.
3) Have the right colors and design, give people a certain feeling about you and what you do.
4) Be at least somewhat unique, goes hand in hand with 1).
If I had the choice between a great logo on a white background html page with regular writing vs. an excellent design without a logo, I would pick the logo anyday.
A great design might bring you bragging rights, but a great logo can actually help build your business.