Sometimes you need to find out the exact Joomla version for a website you don't own for a decent reason - like you are a freelancer and need to provide an estimation for a job on the respective site.
Sure, you can ask the owner ;) but that's the easy scenario.
Fortunately there are relatively easy - and legitimate - ways to find it out, unless the owner of the site hasn't blocked the access to this information. In my personal experience (believe me, I have some) this is rarely happening.