Power series plug-ins you can do certain similar things in Excel as with Power BI and in Power BI you can do certain types of calculations like what you can do in Excel, I would say that the intended purpose of the tools are vastly different. Excel is your traditional spreadsheet program with a very long history which leads to a vast array of features. Traditional spreadsheets allow you to reference individual cells and do all kinds of calculations. Without the power series of plug-ins though there really is no capacity to easily build complex data models, really you are just dealing with essentially unrelated tables of information.
By contrast, Power BI is really focused on data ingest and building potentially complex data models easily. In Power BI, you can easily relate separate tables to one another. You can then create a wide array of visualizations for that data and this is focused toward visualizing aggregations of that data in your data model.