In an ideal situation, a fitness routine should consist of some form of resistance training, cardiovascular training, and mobility or flexibility training. However, if you're lifting weights correctly, you can get plenty of cardiovascular benefits from it. Cardio is the least important of the three, but it can help improve your endurance and overall fitness. If your Vo2 max is poor and it's impacting your strength training, cardio can help. It's not the dominant force in your training, but it contributes to your overall fitness. Cardio can also aid in helping your endurance, getting through some of these sets that you might be in a hypertrophy phase where you're doing lots of reps.