Many times when designing a website, I receive logos that are not in vector format. For those who may not know, vector images can be scaled infinitely, without losing quality, whereas a normal bitmap image will lose quality as it is scaled up, and even just saving the file repeatedly can lessen the quality.
In most cases, logos are originally designed in vector format, in programs such as Adobe Illustrator. They are later converted to bitmap images which get passed around and the original vector file gets forgotten.
We designers need those vector files, otherwise we will need to retrace it which can take some time to get it looking perfect.
This rule of thumb does not only apply to website design, in fact, use of a vector image is most important for using in print or advertising.
As you can see, a vector image will always remain at the highest quality, no matter what size. So be sure to keep this in mind when creating any design that will need to be scaled.