Because if you are generating traffic to a particular website in order just to sell your stuff one time to a customer, then you are missing out on future sales from that one customer that you sweated to drive to your website in the first place!
Practically every newbie, (me included, what a dufus!), makes this mistake. Under appreciating the capacity of monster income that can be generated by promoting yours or others products or services to a repeat customer list that you own.
If you have a list of only 1000 subscribers and manage to convert them at an easy 1% that is still 10 sales X $97 priced offer = $970 from one email that probably took you 15 minutes to write!
So you can see that out of all the ways and means there are of making cash online. Building your own hot targeted list of subscribers is a must and a serious secure long term business asset.