Aerobic or Cardio Intensity is very important to improve Stamina or for Fat-Loss as well.
It is observed that many do the cardio on treadmill or elliptical at a leisurely pace, Talking to each other or watching social media on mobile phone.
If your workout does not challenge you, it will not change you.
Rule of Thumb is "If you are able to Talk Comfortably, means you are not working-out hard enough".
Always check on your Workout Intensity!