Flower-shaped lens hood that came with my Nikon D7000 ?
 

Do I need this? what is it exactly for? it would add a bit of bulk to the lens & make it a little harder to store & then access, so far I have not used it.

Lens hoods block side lighting that can cause unwanted glares and reflections on your images. You always want to use the hood that was designed for that lens, and therefore came with the lens. Only in rare situations where the shade is proven to cause edge/corner darkening (the technical term is "vignetting") do you want to remove it. These days, with new lenses, vignetting is somewhat rare -- especially due to the crop factor of DSLRs. Most DX lens vignetting tends to be caused by glass fall-off anyway.

If you need to further compact your setup, turn the hood backwards and bayonet attach it. When you want to use it, flip it back again. Be careful -- don't break the plastic tabs on the bayonet mounting! I often store my 80-200 hood backwards, but without bayoneting it in -- it's loose, for quicker bag withdrawal and shooting. But you have to be careful -- you don't want to break it by dropping it on the ground, because you forgot it was loose. Replacing hoods can be pricey. (And you NEVER want to use a generic hood.)