Reza Rad explains why the Power BI Service is useful:
The Power BI toolset comes in many shapes and forms. There is a Power BI Desktop, Power BI Mobile app, Power BI Report Server, and Power BI Service (and some other applications and components too). The questions I hear from the new users of Power BI are; Do I need to have an account for Power BI? do I need to use the Power BI website for creating visualization etc.? What is the Power BI website or service, and what is its usage? If I can do the reporting using Power BI Desktop for free, then why would I need the service? In this article and video, I will answer all of that.
Click through for a video or for the article explaining the purpose behind the Power BI Service. Having done work with places using Power BI Report Server and places using the Power BI Service, I will say that the latter takes more work to get corporate-compliant but offers a whole lot more.