The short answer is “How do you know that it will save your life?”
The longer answer is that there are regulations and standards that require certain electrical protective equipment to be tested at regular intervals.
Damage can occur during use (and even in storage, in some cases) and that damage can potentially affect the electrical insulating properties. So, you have two options: either throw them away and replace them, or send them to a qualified lab for electrical testing (learn more in Hammering Home the Importance of Selecting a NAIL4PET Accredited Test Lab).
The lab is a much less expensive option. Throwing away your rubber gloves when you could re-test and
potentially re-certify them by a qualified test lab is like throwing money down the drain. Why spend $200 every six months to replace a pair of
high-voltage gloves when you could test those gloves for less than 10% of that amount without sacrificing safety or compliance?
You will, however, have to replace the gloves if they fail the test. It is neither safe nor a good practice to repair rubber insulating products. Still, replacing only your damaged rubber insulating gloves is much more affordable than routinely replacing every pair instead of having them tested (find out Why You Should Give More Thought to Rubber Insulating PPE).