How and why does Ariel save Alonzo and gonzalo

Ariel, however, hears to conspirators plan, and wakes Gonzalo with a warning of the danger he is in. Ariel intends to let Prospero know that the conspiracy has indeed been formed as he wished, and Prospero in turn will try to keep Gonzalo safe, out of appreciation for his past help in preserving the lives of Prospero and Miranda.