People on the web are in a rush to get what they want. Click they come and click they go. You have a limited amount of time and words on the screen to get your point across.
When people view a web page, they scan it for the information they want. They will scan across the top of your page reading the content. Then they will scan down and read across again. If they scan down further, they will likely scan across the page an even shorter distance.
You need to get your main information across in the first few scans! It is also helpful to use a few tricks to make sure your main point jumps out.