The AE guide says
Reduce Project
Removes all unused footage items and compositions from the collected files when the following options are chosen in the Collect Source Files menu: For All Comps, For Selected Comps, and For Queued Comps.
For Selected Comps
Collects all footage files and proxies used in compositions currently selected in the Project panel.
I assume selecting "For Selected Comps" will collect only the footage files and proxies used in the selection composition as the guide says.
Then what is the point of having Reduce Project selectable when choosing "For Selected Comps" since selecting "For selected Comps" itself will be enough to only collect the files used by the selected comp?