If you want your shelves to stay up you need these!
The 12" position shelf brackets are for use with the following:
12" (30.5cm) deep Linen, Shelf & Rod, SuperSlide & CloseMesh shelving
16" (40.6cm) deep Shelf & Rod shelving, with hang bar 12" (30.5cm) from back wall.
16" (40.6cm) deep SuperSlide shelving, when using a hang bar.
The shelf support brackets are needed to support the shelf and therefore you would need one every 36" (91.4cm) along the shelf's length, and also one at each end if you are creating 'open ends' or floating shelves, where 1009 - shelf support poles, or 974/980 - wall brackets have not been used