1) Let's say you have a music band similar to Radio Head. You could use the Gather from User feature and gather all of Radio Head's Subscribers (which you will then send Friend Requests to).
2) Let's say you have a comedy website geared towards college/university males. You could simply use the Spider Search feature and gather only males in the age range of 18 to 24.
3) Let's say you sell golf products. You could use the Gather from Search Results feature and do a search for Golf. This will give you a list of people who have recently posted a YouTube video about golf and are all potential customers of yours.
4) Let's say you're a hip-hop musician. You could use the built in browser to navigate to a popular video similar to yours ('New York' by Jay-Z for example) and click to view all of Comments on the video. Then, use the Gather all from Current Page(s) feature in Tube Toolbox and all users who have commented on that video will be gathered into a list.