Prototypes can have different goals and different fidelities. Some are early samples to test ideas quickly, some are high-fidelity to showcase details of behavior or interactions.
Even in the wireframing or sketching phase of designing a prototype can sometimes be useful. Getting a feel for how the product would work early on means being able to adjust early on as well.Adjusting early on is often more efficient and cheaper in the long run.
You can also use low fidelity prototypes to test concepts with the end user. A lot of the time a high-fidelity prototype would be used for usability testing, but when the goal is simply to testa navigational element or copy for example, maybe a low-fidelity prototype will do the trick.
