I have been exercising my whole life. Since weight training, I try to do cardio 2-3x a week. Does it "do" anything? I really don't think so , but I generally feel better on days i do vs days i dont.
Same thing for "stretching" which I also do very little. I like doing light lifts for the stretch and lubricating the old joints. Again I feel generally better when I do these than when I take a complete day off.
Cardio, btw, is unbearably boring. Interval training, say 5-10 intervals goes fast. Lately, however, I just grind out 20-30 minutes while picking fights in E&P.