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