A product requirements document is a document which fully defines the purpose of the product or feature. It includes details on how the product should be built, and how it should be supported to empower you and your team to build a successful product.
Having a strong PRD does not guarantee that you'll have a strong, engaging product. However, having a weak PRD almost guarantees that you'll have a weak product.
Your PRD should be used to define the following:
These are key questions that your requirements document should answer. Similar to wireframes, your PRD should leave no questions in terms of how the product should be built.
You might hear this question when working with new product managers:
Do I really need to spec this out? I have a bunch of things I need to get done, so I want to save some time. My engineer is right here, and we already put together our plan on the whiteboard. Isn't that good enough?
As you probably know, the answer is, "No."
A PRD's primary purpose is to help you communicate the key parts of your product or feature and how they are supposed to work. It allows you to communicate this once, so that you don't need to repeat or clarify this to each of your team members. Everyone in your organization should share the same knowledge of how your product works after reading your PRD. And because it's written in print, there should be no, "He said, she said" when the product actually launches.
Your engineers use your PRD to implement the feature according to your requirements. Your designers use it to create what you asked for. Your sales and marketing teams use it to write materials that describe the key benefits of what the product or feature does. And senior management uses it to know exactly what you're building. PRDs are not just useful for you; they improve the work of your whole product team.
So, if anything, writing a PRD will actually save you tons of time. It will allow you to communicate things once instead of doing it multiple times with several different people. More importantly, it sends a signal to your product team that you care about keeping everyone on the same page.