When is rebar used and what exactly does it provide?

Rebar serves several purposes. Dowels and longitudinal strands are simple means of typing together new concrete with existing so that they move together rather than independently. Rebar grids, commonly at 12”-24” on center are placed in load bearing slabs to provide tensile support. Concrete has high compressive strength but low tensile strength. The inclusion of the rebar compensates for the tensile strength deficiency, more broadly distributes loads, and prevents vertical displacement when/if cracks develop.