Unless your training is some how related specifically to something that you need to train for such as a sport or activities you plan on doing regularly in your life, I would have to argue that it isn't necessary at all.
There really is a lot of misconceptions on this topic. To begin with, "cardio" isn't even a real word and when people do use it its meaning remains rather vague. Anyhow, let's just realize that the term is typically associated with exercise that gets the heart going. But let's also be real. You can't separate the cardiovascular system and workout on that alone and not the rest of the body. The cardiovascular system is there to support all movement that we do. And that includes weight training.
On top of that, people typically think of cardio to be exercises such as running and cycling. Again, there is nothing that magically makes such movements different than anything else that we do. Your heart and lungs can't tell the difference if you are running or doing leg presses. In fact, many other types of exercise will jack your heart rate up much higher than what can typically be achieved from running.
And with exercises such as squats, you really do get a benefit for your effort. Let's compare:
Running (steady state) doesn't do a good job of working your cardiovascular system, it wears you out and cuts into your recovery capabilities, its catabolic in nature, and it takes time out of your day.
Squats build muscle and strangth (are anabolic), they can get your heart and lungs pumping real hard, and while they will also wear you out and take some time out of your day I would definitely say they are worth the time. Examples of other good exercises are strongman events such as tire flips and farmers walk.
Another thing to note is your typical "cardio" type training simply isn't needed for good health or fat loss. I never do it. I have proven in my life that fat can be lost without it. I happen to be quite lean. I also went to the doctor last week and turned out to be quite healthy. My blood pressure is healthy and my resting heart rate is lower than average which is an indicator that I happen to be in good shape. The nurse asked me if I was a runner and my reply was that I only lift weights twice per week.
All that "cardio" stuff is nonsense. And once you look into it you will realize it. Your time will be better spent doing anything else, resting, spending time with your girlfriend or family, or what ever.