The reason auto manufactures add stop leak to the cooling systems is, when you build millions of vehicles, and when millions of cooling system parts such as radiators/heater cores are supplied by suppliers/vendors, not every component is going to be perfect. So, the stop-leak is added to prevent any possible leaks.
When I saw a stop-leak being added to new Ford Rangers at the old St Paul plant, I asked the plant manager why.
He looked at me and said, “We’re here to make trucks. Not repair them.”.
Tester