Software faults that lead to failures differ from hardware as they are not random or the result of wear out. They are embedded into the code either through coding errors or because of errors/omissions/ambiguities in software requirements. In safety-critical software, which is rigorously tested, faults are mostly due to requirement issues, and much less frequent due to coding errors. Therefore, V&V of software requirements is at least as crucial as V&V for hardware, if not more so.
Most failures are the result of the code that has not being designed to deal with certain (mostly rare) events: conditions and inputs. Moreover, it is in the requirements that mitigations for failures are listed. For serious failures, multiple (redundant) mitigation strategies are required. A safety-informed requirements V&V focuses on these types of omissions.