For years, there have been a number of reasons why you would want to add nofollow tags to select links on your site. With Google putting such a focus on cleaning up search and penalizing sites for "unnatural links" lately, nofollow is an absolutely essential publishing tool, especially if your site includes guest or sponsored posts. It is simple to add and requires the most basic of coding skills (pretty much identifying the link and pasting in).
Here is a very handy infographic from Danny Sullivan and his team at Search Engine Land that goes over what the nofollow tag is, how to use it and when.
Take this stuff seriously folks. You don't want to wake up one day and find out Google has put your site in an indefinite time out!