If you work out for half an hour and then get off the treadmill, you feel great and accomplished. This is a manageable amount of stress.
But for comparison, imagine if you have to run on that treadmill day after day after day, only getting off for regimented breaks and the bare minimum amount of rest required, and you are only allowed two weeks of vacation a year and then it's back to the treadmill. And if you ever stop running on the treadmill, you lose your house and ability to feed yourself and provide care for your family... It gradually wears your soul down to a nub.
Not to mention: if, in a moment of weakness, you slow down on a treadmill, braking braces on your legs kick in and start giving resistance, making running harder. Thus any kind of slack or mishap becomes a permanent handicap.
Useful stress is something you can relieve through action. You study and learn the thing, and pass the test. Or you have that difficult conversation. Once you go through it, you feel a sense of pride and accomplishment (sorry).
Useless stress is what you feel when you’ve think you’re going to lose your job, or a loved one is sick, or a brush fire is burning a mile away from your house. It’s out of your control and the stress you’re under can’t be harnessed in a useful way. It just sucks.