Scrum is a lightweight framework that can be incredibly flexible, efficient, and powerful, but, much like a vehicle, the best body and designed frame will get you nothing without a powerful engine to move you forward. There are two main important roles that exist on every Scrum team: the Product Owner(PO) and the Scrum Master(SM).
The PO is the business representative of the team. They're not part-time team members. They show up every day, because they're contributing to the final product every day.
They review all the work the team completes and either accepts it or asks the team to make changes to ensure the highest value is being delivered. Earlier, the business person was presented through requirements documents that were rarely, if ever, updated.
On a Scrum team, the PO is always ordering the work and ensuring that the team members clearly understand the details of the request, but that's only one part of their job. They're also interacting on a daily basis with the stakeholders.
It's not enough for the PO to interact with the team, they must also be in tune with all the changes that are occurring in the business context. As a result, the PO is the keeper of the product vision. He or she defines and manages the backlog of work to be done and the prioritization of those work items.
The Scrum Master is the most visible spokesperson for the team. Scrum Masters value transparency. They'll devise charts and boards to share the team's progress with anyone who's curious or interested in knowing how they're doing.
They're also the first escalation point when something gets in the way for the team. The Scrum Master will work to remove any blockers until they're out of the way and the team can continue on. While the Product Owner keep eyes on what needs to be done, the Scrum Master focuses on how the team does the work.
One more thing, The Scrum Master also holds the team accountable for their commitments to the Product Owner. They show trends in team performance over time to help the team improve their processes and practices. As you can see, each role is absolutely critical to getting the framework to function properly. If you're lacking one of these roles, Scrum will be far less effective than if you have them both.