That is subjective.
Some people just prefer to only do LFR and be done. They consider that they have done everything, because they don't care about something harder and feel like since they have killed the boss in LFR, it's dead in general.
Some people prefer to try something harder. They consider they have done everything only if they have killed every boss on mythic.
And, of course, there is everything in between. The difficulties exist so that you can do what you prefer/have time for.