Well, first think of how does a user enter your website; via search engines, directly or from a link? If the user, me for example is entering your site directly without knowing what to expect or look for, it gives me a short pause to think about why I'm really there. You want to be able to direct the user of where you want them to go and eliminate that chance of may be leaving the website. This is when special promotions or other call-to-actions give the user the way in to go deeper into the website.
So for example if I enter the website and I glance at your featured products and I don't find right away what I'm looking for, then I will most likely leave. But if for example you had a special Father's Day promotion of say 25% off, I'm more interested to see what I can get, so I continue looking. Do you see the logic? Of course not all users will react the same way, but you want to appeal to the majority. This doesn't say that you have to have a discount all the time, it could be best sellers, new items, group of items under same color/material...
Does this make more sense?