As a whole, the business intelligence tool amazingly helps you connect various sources of data, clean it up and transform it into a data model for clear Data Visualization (graphs, charts etc.) And share it with other users within your organization.
But how you use each element of Power BI may depend on what is your role within a team or a project at a given point in time. For example, you may be using the Power BI service. But a business report creator within your enterprise who does a lot of number crunching might be using Power BI Desktop extensively to create reports, then publish those reports to the Power BI service, where you can view them. Similarly, another associate, in sales, may require using the Power BI phone app to monitor progress on sales quotas, and to fill in new sales lead details. Developers also use Power BI APIs to push data into datasets or to embed dashboards and reports into their custom applications.
And all this in 2 simple steps to make your life much easier.