How do you ask others for link opportunities? Most of the time people are only interested in either reciprocal links, or them providing guest posts on my site (when I reach out). And I can’t imagine if I did a round up post getting many inbound links. People would be thrilled that they had received a link, and wouldn’t create a reciprocal link to destroy the value.
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.
For example, if a swimming pool business is trying to rank for "fiberglass pools" -- which is receiving 110,000 searches per month -- this short-tail keyword can be the one that represents the overarching topic on which they want to create content. The business would then identify a series of long-tail keywords that relate to this short-tail keyword, have reasonable monthly search volume, and help to elaborate on the topic of fiberglass pools. We'll talk more about these long-tails in the next step of this process.