There are a few reasons why a unique logo is an important factor in a website.
1) It establishes a brand for your website. People come to recognize your website and remember your website sometimes by the logo alone.
2) It adds an additional pazazz for your website.
3) You can use your logo as a favicon sort of like what WMS has done with this forum.
There are probably more reasons why a unique logo is important for a website, but I can not think of anything else at this time.