When comparing the features of the Online Client with the Desktop version of Power BI, there is one very obvious difference, there is no way to create a data model in Power BI online. It is not possible to create a data model using the online client. The online client is designed to connect to an existing online source such as Sales Force or Azure DB. If you are using an existing model, there is no need to create one. When using the enterprise gateway, which uses an on-premises database such as a SQL Server, SSAS or Hana, the data model is contained within the database exposed via the enterprise gateway, so again no reason exists to create a data model. Report creation can occur either using the online client or desktop as there is compelling technical reason that I am aware of which would determine where the report is created.