Not all linking is created equal and that is why having a experienced SEO firm will benefit your website in the long and short run. They will be able to get those qualified guaranteed links back to your site with out wasting your SEO budget. Experienced SEO firms now the best places to list content and link back to your website and get that guaranteed link back.
Good quality linking comes from highly ranked related industry sites and then linking back to your site. About 3-4 years ago, a Google update slashed the value of links from unrelated sites. An example can be a real estate website linked to a high tech site. There is no logical relation there. Before the update, a link was weighted equally. Now, unrelated links are not counted and all related links are weighted based on anchor tags, the PR of the page their on, and the page they link to.
Lastly, knowing how many links to build in order to get to the front page and then above the fold on the SERPS (search engine result pages) is key. You don't want to keep on building links when your already #1 on the front page of Google. The experienced SEO firm will move on to a new set of keywords and continue to grow your websites overall keyword portfolio.