You are human. And you will make mistakes. You may do or say things that hurt your spouse. You might not do it on purpose. But it may just happen. Not because you are a bad person, but because you are human.
When someone feels wronged by another person, the wrongful act stands as a barrier between them, and the relationship is somewhat fractured. If the relationship is important, then both really want a reconciliation, perhaps even more than wanting justice or revenge.
A sincere apology makes reconciliation possible.
What happens next can make all the difference. A sincere apology can bring that reconciliation. If that does not happen, then the wronged one will continue to feel hurt, the wedge is still there and grows. Depending how big the wrong was and how often it happens, the relationship can really totter. They may even seek or demand justice, understandably so.