If we don't get a vaccine, covid will kill around 600,000 people in the UK in the end.
If we do get a vaccine, that number will be lower, depending how many have become infected before we get the vaccine. So, the lower the number of people infected early, the better ... many more people will survive, if we get a vaccine.
Even if we don't get a vaccine, the rate at which people become infected is also an issue. If it's a low rate, the NHS is not overwhelmed. More people will survive, both covid and other ailments that put them in hospital.
On the other hand, if it's high rate, and the NHS is overwhelmed, more people will die, both from covid and from other ailments that put them in hospital. These might be "ailments" that could affect anybody, young or old, like a car crash ... if the intensive care unit is full of covid patients then car crash victims may not survive when otherwise they would have.
So the thinking is that if you want as many people to survive as possible, you keep the infection rate low and make it take as long as possible to pass through the entire population. It gives us longer to get an effective vaccine, and stops overwhelming the NHS. To do this, we need to find a way of living that keeps the infection rate down while allowing the economy to continue.