Is there a reason Finder opens separate Info window for group of files/folders I have selected?
I know that I can use Summary Info after pressing Control, but what is the
  use case for default behavior?


It's working as expected.

The action that you select is operating on each file individually.  If you consider the other Finder functions (i.e. when you right click after selecting multiple files/folders), each function operates on each file individually.


Rename - renames each file/folder
Duplicate - duplicates each file/foler
Quick View - opens dialog for each file/folder
Compress - adds each file/folder to a zip archive
Share - shares each file via messaging, email, etc.


Look at it in this manner - Finder doesn't do a single operation over many files, it does one operation on each file for as many files selected.

The reason Summary Info is there, is for this exact reason, so you can get the aggregated info of many files because the default action is to operate on the folders/files individually.