The first reason, and the most obvious, is the buying cycle. You want to totally dominate your market which means that you should totally dominate the buying cycle. The only way you can effectively do this is with a list.
Statistics show that people need to be exposed to a product at least four or five times before they purchase. People are surfing the net all day long, so how do you expect them to remember you? How will you get them back to your site to expose them to your product?
How many times have you seen a site and thought of going back and taking another look but can't remember where it is? This is the number one reason you want to be building a list.
If someone has been sent some follow up emails then the chances are you are building your brand in their mind. They get emails and read them, reminding them of who you are and what you are selling. Once they are ready to purchase, you will be their first port of call.
Another reason is that you are likely to expand your business online. You then have a list to promote your new products to already. I know that when I send out to my list on the launch of a new product I know that it will do well. I know that they are proven buyers, I know that they are interested in what I offer them and if I have built my list properly that I have a relationship with them. They will fall over themselves to get hold of my next product.
I can go on and on about the benefits of building a list, but with the above points I'm sure that you have realized that this is one of the most important parts of building your online business.