Explanation
The advantages of a Product Owner sharing a clearly defined Product Goal with the Scrum Team are:
* It provides a good overall direction so Sprints will feel less like isolated pieces of work. The Product Goal is a long-term objective for the product that guides all the activities of the Scrum Team. It helps the Scrum Team align their efforts and focus on delivering value that contributes to the product vision.
* It helps the Scrum Team keep focus and they can weigh any decision against the Product Goal. The Product Goal serves as a filter for deciding what to work on and what not to work on. It helps the Scrum Team prioritize and optimize their work based on value and impact.
* It is easier to inspect incremental progress at the Sprint Review. The Sprint Review is an event where the Scrum Team and the stakeholders collaborate on the product and inspect how it meets the Product Goal. The Increment is a concrete and usable outcome of the Sprint that provides a measure of progress towards the Product Goal.
Other options, such as helping the Developers estimate the date that the Product Backlog will be complete or being not mandatory in Scrum, are not valid advantages of having a Product Goal. They may reflect a misunderstanding of what a Product Goal is or how Scrum works.
References:
* [Scrum Guide], page 10, section "Product Goal"
* [Professional Scrum Product Owner Training], page 6, section "Product Vision"
* [Professional Scrum Product Owner Training], page 7, section "Product Backlog Management"
* [Scrum Guide], page 13, section "Sprint Review"
* [Scrum Guide], page 10, section "Increment"