Assertions are a necessity in systems programming, especially when you're talking about OS kernels.
In one software project I know of, assertions are used to ensure that situations that should never happen never do.
This could be as something as harmless as incorrect (but harmless) usage of a device driver interface, or slightly more terrifying cases that could cause data loss if they occurred.
Before you claim that it should have more unit testing, it actually has mountains of it; again, the assertions are a safety net that prevents even worse things from happening without significantly impacting the performance of a production system.
In one software project I know of, assertions are used to ensure that situations that should never happen never do.
This could be as something as harmless as incorrect (but harmless) usage of a device driver interface, or slightly more terrifying cases that could cause data loss if they occurred.
Before you claim that it should have more unit testing, it actually has mountains of it; again, the assertions are a safety net that prevents even worse things from happening without significantly impacting the performance of a production system.