1. Because they are normally at least 1/3 of the search results that show up on the first 2 pages of Google. If you don't do them, or don't do them properly, then you will miss out on 1/3 of that traffic to your website.
2. You will also miss out on the traffic that comes from the listing sites themselves when people are searching for your service on those sites.
3. Finally the listing sites have a high PageRank so listing your website on them will give your website an SEO boost with many new and powerful backlinks from these directory sites.