You'd most likely see title attributes applied to abbreviations, if they weren't written out in full, i.e. WWW: <abbr title="World Wide Web">WWW</abbr> regarding links usually they shouldn't be needed as the link usually should be self-explanatory.
Though sometimes where space is limited in a navigation bar; <a href="mailto:firstname.lastname@example.org" title="Contact the XYZZY Manager via E-mail">Contact</a> might be easier to understand than just the link that may trigger an email client, etc. That wasn't a good example though should give an idea. Recently we had a thread with how some AT devices handle the attribute: Too Much Accessibility: titles titles titles everywhere.