In addition to all the above points regarding blocking out unwanted light, lens protection and sex appeal - the lens hood is also unparalleled protection against lemurs - and other muddly/sticky pawed/nosed creatures:
for macro it depends a lot on the lens - my 150mm always has the hood attached and hardly affects working distance - whilst the 35mm and 70mm both have quite recessed front elements, so on the shorter lenses you don't need the hood because the build of the lens already provides a small one, and further at the close distances they do affect the working distance noticeably