Two reasons why you build backlinks: for direct traffic and for SEO - SERP.
You could actually hit them both by doing the following:
Directory Submissions - targeted links at high
PR directories
Blog and Forum participation - to high
PR and frequently crawled sites
Social Bookmarking - to high
PR SB sites
Press Releases - for traffic
Classified Ads - for traffic
Article Marketing - submit to high
PR directories, could also serve as link baits
Link Exchange - with relevant and authority sites
Link Baits - for organic linking
Etc. etc.