Do interfaces have any purpose besides achieving polymorphism and multiple inheritance?

I am trying to understand what the benefits of using interfaces are so that I can know when and how to use them. Most sources on the internet are relatively surface-level, explaining how interfaces work but now why to use them, and when I look up the titular question, I don't get any results that tell me whether the purpose of interfaces extends beyond polymorphism and multiple inheritances.

My reasoning is that if an interface were inherited by only one class, it would be useless, and when an interface is inherited by multiple classes, it makes no difference unless it is used for polymorphism, and the only thing that makes implementation different from extension is multiple inheritances.

If I knew for sure that their purpose was limited to this, I would have an increased confidence in my design decisions, and if I learned of a purpose outside of this, it would fill a serious gap in my knowledge. I have used the design patterns tag because there is perhaps a design pattern which makes use of interfaces in a way that is distinctly beyond mere polymorphism or multiple inheritances.