After having a stress-induced LAD heart attack at the age of 46, my cardiologist gave me the "you need to reduce stress" line and I responded with "Oh, so you're not going to send me a bill for that stent in order to help me reduce my stress?" Apparently he believed that my heart could handle the stress of that bill after all.
And yet, it works a lot more often than you'd think. Just having someone who you perceive to be in some kind of position of authority tell you that it's okay to relax and calm down can have a significant effect on one's stress levels. And it works even better when you're not having to worry about how much listening to the doctor tell you to reduce your stress levels is going to cost you financially.
While doing my PhD at a top-tier college, I went to their health center for psychological counseling and said "I can't study and concentrate well, I don't sleep enough, I don't do enough exercise, I eat crap, and I drink too much." The expert told me that I needed to study and concentrate better, sleep well, exercise more, eat healthily, and drink less.
If people could just “reduce stress” on a whim, they would. Having a doctor tell you reduce stress will actually increase stress.