The images we are used to seeing are called “bitmap”, and they essentially consist of a grid of colored pixels: when they are enlarged past a certain limit, they become “blurry”. Vector images, on the other hand, are managed by mathematical functions, and can be scaled up indefinitely without ever losing detail.
Web applications are based on the exchange of data over the network: the transfered files must be small in size, to ensure quick and easy loading and an optimal web experience. Vector images more than satisfy this requisite.
With more and more powerful softwares to handle this type of graphics, ther is really no boundary to the creative possibilities. Vector graphics are natively supported by the CSS format, that manages the appearance of web pages, and it is even possible to create high quality animations using this combination.