Joomla has been one of the most popular CMS for a long time. It powers a huge number of sites. That’s great! The flip side of this fact is Joomla has been very popular for a long time and there are still very many sites that use older versions of Joomla as well as older version of Joomla components. For example, the 1.5.x branch of Joomla (2008-2010) still has a noticeable share in live Joomla sites.
Old versions may work well for your site but they have multiple well known security holes, so they are the low hanging fruit for hackers. Let me show this using a real world example.