Select It makes it easy to open text-links and search Google on text-selection.
You might not believe it but the web is filled with links that aren't clickable. It sounds weird, but it is very common in forums or comments sections where the site owners does not want users to navigate away or to prevent spammy links.
What Select It does is make these links (double) clickable.
Sometimes Select It has a hard time opening a link. In those cases you can select the URL and press CTRL+F. If you select something that is not an URL it will instead be searched on Google.