Unless you're already a master of the topic of your process essay, you'll be offered the opportunity to learn something new when writing one. Even if you are competent at your topic, you'll still learn something new when outlining the required steps in your process essay, which can only strengthen your grasp of the topic.
When writing a process essay, you must break an action into its core steps. This forces you to thoroughly contemplate what you're writing about. The steps you took for granted or didn't think about when completing a certain task must be spelled out for unknowing readers. If you did have any misunderstandings about a topic, you'll likely overcome them when you write a process essay.
Aside from learning what you're writing about, you'll also learn how to communicate in a clear and direct manner. Process essays by their nature must be clear and direct. Leaders in the workplace often must know how to instruct others in a clear and direct way, and writing a process essay builds this skill.
To write a process essay, you also must know who you're writing for. In general, anyone should be able to understand your essay, but each process also has a specific audience. An essay on how to string a guitar, for example, has a specific audience who will know certain words and phrases that aren't used to describe other topics. When writing a process essay, you must know or learn how to properly communicate to your audience.