The main purpose of cardio is to ensure a level of daily movement to avoid age related health issues. That being said though, it is a great tool to use in a fat loss phase to increase caloric output - and hence creating a greater caloric deficit without needing to cut on food.
With cardio you have 2 main options:
LISS - Low Intensity Steady State
Underrated, effective & useful for those who dislike the intense pace nature of HIIT.
Examples of this are: walking, brisk walking, LISS circuit training or using cardio machines at a lighter intensity for a longer period of time
HIIT - High Intensity Interval Training
Most common, effective & useful for those who prefer the faster nature of HIIT.
Examples of this are: jogging, running, HIIT circuit training or using cardio machines at a higher intensity for a shorter period of time or having high intnestty intervals on the machines