Product managers make sure that the features being built meet and support the overarching business goals of the company. They work cross-functionally to determine what to build, why it should be built, and when.
Here's an analogy.
Working with a product manager is like working with someone to plan your custom dream home: You have a vision for what you want, but you also have a budget and a timeline influencing the construction process. Depending on your overarching goals for your home, you may prioritize an extra bedroom over a home gym. Product managers help with decisions like this, and much more, for digital products.
Why does it matter?
Product managers make sure the right product is being built. They ensure you are building strategically for your target market, rather than building custom features for your loudest customer. They help communicate overarching goals between the business and technical teams, and they help you get the most out of your engineering investment by making sure the right thing is being built.
How does it apply?
Product managers facilitate the process of bringing digital ideas to life, and taking existing digital products to a new level.