A couple of easy optimizations, for example, that would be available to a smart compiler processing straightforwardly-written code to use automatic overflow checking, but not to one fed code that uses intrinsics:

1. If code computes x=yz, but then never uses the value of x, a compiler that notices that x is unused could infer that the computation could never be observed to produce an arithmetically-incorrect result, and thus there would be no need to check for overflow.

2. If code computes xy/z, and a compiler knows that y=z*2, the compiler could simplify the calculation to x+x, and would thus merely have to check for overflow in that addition. If code used intrinsics, the compiler would have to overflow check the multiplication, which on most platforms would be more expensive. If an implementation uses wrapping semantics, the cost would be even worse, since an implementation would have to perform an actual division to ensure "correct" behavior in the overflow case.

Having a language offer options for the aforementioned style of loose overflow checking would open up many avenues of optimization which would be unavailable in language that only over precise overflow checking or no overflow checking whatsoever.