Spacers are only placed between data tiles. They were designed for applications like mailing labels. You can also increase the spacing by increasing the width of the data tile. However, if you are printing something like 3-up mailing labels with automatic number of columns, this can result in the width exceeding the paper width, so that only 2 columns are printed. Since the spacer after the last column isn’t included in the report width, that solves that problem. It could also be solved by making the data tile wider and setting the number of columns to 3 instead of automatic (or whatever the exact needed number is).
None of this has anything to do with a summary report. If you want the summaries to be separated vertically, just make the summary tile taller. Summary tiles aren’t separated horizontally at all, they are always vertically stacked on top of each other.
Summary can be included in a multiple column report. But they still aren’t separated from each other horizontally, they are stacked vertically within each column.