Getting links that point to your website is an essential part of seo strategy. Without enough backlinks to your site the page rank would not increase. If the pagerank does not increase your traffic would not increase. If you are running a business then traffic is the ultimate factor to convert your visitors to customers. The search engines count the amount of backlinks you have gives a rank. Having more and more backlinks will increase the sites popularity and will generate more traffic from the search engines.
One essential part of seo can not be forgotten when building backlinks. That is deep linking. Deep linking is linking to individual pages instead of only to the home page. If you only would link to the home page the spiders will 'crawl' your site and will see this as a shallow page. If you deep link to individual pages the spiders will 'crawl' the site better and will store your data on the search engines.
The directories would not give links to the internal pages and it is possible to pay for these links that point to internal page. If you have quality content written on the website it make other web masters link to your site.