Summary

We have a whole chapter dedicated to the subject of UB in C++ and in programs in general. Why? Because this subject is inextricably linked to performance.

First of all, understand that UB occurs when the program receives an input that is outside of the contract that specifies the program's behavior. In addition, the specification also says that the program is not required to detect such input and issue a diagnostic. This is true for the UB as defined by the C++ Standard and for the UB of your own program.

Next, the reason the specification (or the standard) does not cover all possible inputs and defining the results is mostly related to performance: UB is often introduced when it would be very expensive to produce a specific result reliably. For UB in C++, the variety of processor and memory architectures also leads to cases that are difficult to handle uniformly. Without a viable way to guarantee a specific result, the standard leaves the outcome undefined.

Finally...