Marco Russo and Alberto Ferrari share some thoughts:
SUMMARIZECOLUMNS is the most widely-used function in Power BI queries. An important and unique feature of SUMMARIZECOLUMNS is that it determines automatically how to scan the model to produce its result. Indeed, when using SUMMARIZE, GROUPBY, ADDCOLUMNS, or any of the more basic querying functions, developers must declare the source table to perform the grouping, as well as the group-by columns and the measures to add to the result. On the other hand, SUMMARIZECOLUMNS requires only the group-by columns; there is no need to provide the source table, which is the primary ingredient of any query. SUMMARIZECOLUMNS figures out the structure of the result by itself, using a sophisticated algorithm that requires some understanding.
The pair do have a whitepaper available on their premium (paid) service but even the free post contains a lot of detail you’ll want to check out if you use DAX.