It is important to understand the significance of different roles and how they work with each other towards success. Let's first understand what does it mean to manage a project vs managing a product.
Managing a Project
In order to ensure a project completes successfully, on time, within budget, it is important to ensure the goals of the project are well defined and understood by the team. The right people are assigned to work on the project, and the right resources are made available. The sequence of tasks, dependencies between the tasks are clearly defined and understood. Any risks to the project are continuously identified and there is a plan to mitigate these risks. In short, this is the role of the project manager. A project manager feels successful in his or her role when the project is through to completion while achieving the stated goals and objectives.
Managing a Product
Managing a product is really about understanding the entire lifecycle of the product, starting with the rationale for even building the product in the first place. Who do we intend to build a product for? What problem does it solve? What will the product do and achieve? This is typically known as Customer Discovery, and involves learning more about prospective customers, their needs and challenges. This leads into defining the requirements, specifications, user stories so that the broader development team understands the customer context, so that we build the right product for the right people. This is the primary responsibility of the product manager. A product manager also works closely with marketing and sales teams to enable them to position and sell the product effectively. Additionally, a product manager has to ensure the product is preferred over the competition and other alternatives the customer might have, and define a roadmap and strategy for the product.
As you can see, the disciplines of project management and product management are quite different. In a nutshell, product management is about defining the WHY of the product, while project management is about defining the HOW of the product. Project managers and product managers need to work closely with each other for the success of the product.
One interesting aspect I have seen in my experience is that project managers are successful when they tend to act like product managers. However, when product managers tend to act like project managers, they end up being unsuccessful because it drags them into the details of managing projects and takes them away from managing the strategy and the business of the product.