This attribute window button toggles the display for attributes which contains an array of values. If you use it, ALL multi valued attributes in the current list are show expanded or collapsed:
This was the collapsed view, here comes the expanded display:
Please note that there is a quick and easy way to switch the multi valued display for a single attribute in the list: Just press the SPACE key on your keyboard or use the context menu (right mouse click) and the option Show Multivalue as Array / Show Multivalue as Singe Line.
The Set multivalued attributes button is not available if the attribute window is in text output mode.