If you are using Power BI Desktop or Power Pivot in Excel 2016, you should learn when and how you can use GROUPBY instead of SUMMARIZE. The SUMMARIZE function is very powerful and internally very complex, so it’s easy to find scenarios where you get unexpected results or you have performance issues. The new GROUPBY function (also available in SSAS Tabular 2016) can be a better choice in a number of cases, even if it’s not the same and, for example, it does not “enforce” a join as you can do using SUMMARIZE (see here for more details).
I recently wrote an article about one situation where GROUPBY is absolutely the best choice: when you have nested grouping. An example is pretty simple: you want to SUMMARIZE the result of another SUMMARIZE… well, it’s not possible, but you can do that using GROUPBY.
Once you get used with GROUPBY, I also suggest you to check your skills with the DAX Puzzle about GROUPBY we published a few weeks ago. And if you alread solved it, try the new puzzle published less than two weeks ago about “last date” – not related with groupby behavior, but still good food for mind!