Web users tend to browse sites based on their reading habits. The F shape is logical because reading starts at the top left (for LTR languages). Then, when a line ends, readers need a bit of orientation to find the beginning of each next line. Little breaks like headings, paragraphs, images, quotes and their white space give new focus points, a new place to drop into the text. This is where readers orientate again. All this reading behavior leads to a F shape scanning pattern.
Pull the eye
There is a F shape because the content is structured in a F shape, the eye will follow that shape. But the F shape structure isn't true for every design. You can guide the eye with composition and visual mass (color, size, etc) and pull the eye to any position.
Constructivism has great examples:
Ladislav Sutnar's design for a 1932 translation of George Bernard Shaw's Captain Brassbound's Conversion . Source: http://www.sil.si.edu/ondisplay/czechbooks/intro.htm
There are other ways to pull the eye. Attention is also drawn by images of people and faces.
Scanning a page has also a strong relation with the task at hand. Think about a typical F shape layout and the task to find a phone number. The eye will ignore the text block and will scan the edges (last items in the navigation, column, footer, etc).
Use priority placement for your main message in your design. The most prominent place doesn't have to be top left. It can be anywhere. It's about visual hierarchy.
Advertisers pay for space to get their message across. Again, this doesn't have to be top left. In speed skating it is the upper left thigh (lower right side for the viewers). This position has most exposure and therefor most value.
"The speed skating world believed the right leg, at the stand side of the track, is best for sponsor logos. Not true: the left leg is gold."
The F shape pattern doesn't apply to all designs or all tasks. You can pull the eye to any position. Designers should consider this power and use it to their advantage.