Usually after assembling models, particularly plastic ones, we are left with a handful of spare parts. Don't get rid of them as they may one day become a glorious addition to a new terrain piece or converted model
It is worthwhile to collect not only bits from miniatures but also characterful "junk" such as oddly-shaped containers, electronic equipment parts and anything else that has a potential to become a modelling project.
PS: Now in the 2016 the bits box is actually a collection of several carton boxes and small plastic containers with dividers to keep various types of small bits apart. You don't want your dwarven heads mixed up with Space Marine shoulderpads.