You want to learn sing with chest voice because it can really make you sound great.  You can sing without using your diaphragm like you talk most of the time, but you’re going to wear out your vocal chords and start straining.  You can also incorporate falsetto into your voice to give yourself more dynamics.  Chest voice is utilizing other cavities in your body you may not be used to using while singing.  Singing from the diaphragm also gives a different resonance while singing.  Those great sounds coming from chambers in your body so it’s very complex when you consider everything that creates your true singing voice.