A worn belt can generate electrostatic charges that can damage the integrated circuits in the treadmill display. Indirectly, a worn running belt that generates electrostatic charges also creates more friction on the tabletop and therefore overloads the motor - which leads to damage to the PCB.
I have a used belt - what should I do with it?
Treadmill belts are consumables. Like car tires, they should be replaced when they wear out too much. A worn out treadmill belt is beyond repair. It must be replaced. You can replace it yourself or have it done by a fitness equipment service. Replacing a belt is not a complicated matter.
After replacing the belt with a new one, remember to lubricate the surface between the belt and the running board. Lubrication is necessary to avoid excessive wear of your treadmill's consumables. Remember to maintain your treadmill.