If the browser doesn't know the size of the images, it has to wait and download them before it can format your document properly. By knowing the size of each graphic the document can be displayed with gaps left where the graphics will appear. As each graphic downloads, it is displayed in those gaps. The user can scroll around and read the text while the graphics download. There are Web browsers that are text only.
Lynx is a text only Web browser.
Small A crucial area of Web design is graphics. Having fancy graphics is one thing, but users grow tired of waiting for graphics to download. You must learn how to design graphics that are small. A general rule of thumb is to keep graphics under 25k. If there are a lot of graphics on a page then try to keep your graphics 15k or under.