K2 is basically an extended and feature rich version of your default Joomla article manager. It comes with many more features, making life easier for administrators to manage their content on their website. It also integrates with a lot of other 3rd party extensions. Some of the key features that K2 includes are:
- Commenting system
- Tags (handy for a blog)
- Drag n Drop
- Extended user profiles
- Powerful API
So to answer your question simply, it's better to use K2 as it prevents having to install multiple 3rd party extensions such as Community Builder (just an example) for extended profiles and JComments (just an example) for the commenting system.
Have a look at the K2 Site and even give it a test to what what you can achieve with it.