When you purchase a Power BI Premium SKU, you unlock the AI workload, which provides additional AI insights during your data preparation tasks. These AI insights are powered by Azure Machine Learning and Azure Cognitive Services. There is no added cost to your organization to leverage the AI workload since the use of the underlying Azure services is already bundled into the price of Power BI Premium. 

The other key component is that Power BI Premium offers a dedicated capacity hosted by Microsoft for consistent performance.  Dedicated capacity differs from shared capacity where workloads run on computational resources shared with other organizations. 

Dedicated capacity is an isolation of the computation resources for use by a specific organization. This capacity is also how it can provide dependable and consistent performance for an organization's hosted content. The Power BI service makes the best use of an organization's dedicated capacity resources while not exceeding limits imposed on the capacity.