What is the main reason for the Product Owner to be part of the Daily Scrum?
When multiple teams are working on the same product, their Sprints must start and end at the same time.
All the work necessary to achieve the Product Goal, including Sprint Planning, Daily Scrums, Sprint Review, and Sprint Retrospective, happen within Sprints.
The Product Owner has to maximize the value - more features always bring more value. Therefore, the Product Owner has to maximize the number of features in a sprint.