Ask yourself what a layer is and you will answer the question for yourself. You can fit more content on one single page with layers. Lick clicking on a button when you will display a new page, layers will allow the developer to display the information in the same page. This is only a simple answer but layers are very useful and allow interaction between the page and viewer

thankx for ur reply,

I want to ask that how a web page can be design in Adobe n then hw its slices can be transfered to dreamweaver?

Here is a tutorial to follow to let you cut an image in Adobe photoshop and put it in a webpage using dreamweaver. Good luck and have some fun.
http://www.mac-designs.co.uk/help/cut_image.html