Taking a step back, your body makes insulin to signal to cells to take in glucose from the blood. The cells will move a glucose transporter to their membrane allowing glucose to move into the cells from the blood. This is a great way to deliver energy to them and it is part of regulating blood glucose levels. From an evolutionary standpoint higher blood glucose is better than lower, if your sugar goes too low you cant outrun that lion chasing you and in extreme cases your heart can even stop. Meanwhile high blood sugar wont kill you (at least not for many years), even really high blood sugar wont instantly kill you (though it is very dangerous), so its better to go up than down. From this perspective insulin resistance makes some sense.

It has been theorized that insulin resistance historically was protective against low blood sugar during periods of fasting (between hunting meals/gathering food), the last thing you’d want is to eat a few berries after fasting for a few days and drop dead from low blood sugar as all your tissues try to take in more sugar than is available. However today that is not the problem and the insulin resistance we see now is beyond what is “normal”.

As insulin resistance develops your pancreas responds by producing more insulin, the result of this compensation is normal blood sugar. But as insulin resistance gets worse the additional insulin is no longer enough and so blood sugar rises. Foods that would not cause spikes in blood glucose for healthy people now cause spikes. And it doesn’t take much sugar, there are only a few grams of sugar circulating in your blood, going up by even 1 gram is very significant as its a tightly regulated system and over time it will lead to vascular problems which lead to other problems increasing the risk of mortality.

Diet and exercise (or lack of it) are the cause of many of these problems in the first place but they can reverse them too. Eating less simple carbs and more fiber (among other changes) exposes the body to less sugar spread out over more time which allows for the now impaired glucose delivery system to keep blood glucose levels normal.

Exercise increases the energy demand of muscles. When before they were told to take in sugar by another part of the body by insulin, now they do it themselves including through different mechanisms which improve their overall ability to take in sugar and lower blood sugar (even when not exercising). Aerobic exercise like running/walking will increase the amount of mitochondria in muscle cells which means they can use more energy (among other metabolic changes) allowing them to take in more sugar. Strength training exercises make muscle cells bigger, improve their ability to burn glucose anaerobically and increase their total energy needs (even at rest). More lean mass/muscle burns more calories and acts like a glucose sink.

In short you lower the amount of glucose your body has to deal with at any given time and you increase your body’s ability to move it out of the blood with diet and exercise. Not to mention there are health benefits to diet and exercise beyond type II diabetes management, pick a disease and odds are diet and exercise lower the risk of developing it and dying from it.