.....either because the lens focuses kinda close or because it gets a reasonable mag.....Yeah, I discovered that recently, with my 300mm f4L. It has a "Macro" range "|_____|" indicator at the closest focusing point. It really meant that I could use it to take decent close-ups without resorting to the 100mm Macro all the time
I took a beaut pic of a Wolf spider with it last week, that really came up well (the hairs on the legs were tack sharp in the image). It surprised me how good the IQ was for a tele at the extreme focus range.