When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.
Getting free website traffic may not cost you monetarily, but it will require effort on your part. However, the effort you put in will equate to the quality of the traffic you generate. As mentioned above, there is no point in getting more traffic to your website if those visitors are not likely to engage with your pages, convert into leads, or become customers.
Traditionally, defining a target audience involves determining their age, sex, geographic locations, and especially their needs (aka pain points). Check out usability.gov’s description of personas and how to do task analysis & scenarios for more details, or better yet, read Vanessa Fox’s upcoming book about personas related to search and conversion.
Lastly, it's important to remember that paralysis by over-thinking is a real issue some struggle with. There's no pill for it (yet). Predicting perfection is a fool's errand. Get as close as you can within a reasonable timeframe, and prepare for future iteration. If you're traveling through your plan and determine a soft spot at any time, simply pivot. It's many hours of upfront work to get your strategy built, but it's not too hard to tweak as you go.