Hello Harry,
A lot of people use SSL Certificates on their websites, as they become easier and easier to gain access to, often for free.
Most sites don't 'need' SSL, however google penalises sites without SSL and numerous major browsers are looking into showing a security warning for sites without SSL.
Even my blog with very few readers, and only myself as an account holde, and it has complete SSL Throughout as it doesn't effect the site itself and improves user experience as there's a lovely green/black lock.
Rather ironically, this forum only just got SSL recently, and this page won't show a lock. Simply becuase there's one or more websites/links that aren't protected by SSL. Using Internet Explorer will/should cause a security warning to appear saying that some unsecured content has been blocked.
Hopefully I answered your questions throughly, if not feel free to ask further questions.
Best Regards,
Kieran Cairns