Adding rebar to deck footings and piers?
Concrete does not have very good tensile strength. Rebar is laid in such a manner that it adds tensile strength in the concrete.

Depending on the size and weight load the pier/footer assembly is expected to support and the length of the pier, it could be reinforced in the following manner.


A rebar circle in the footer to prevent weight spread from cracking it   
Vertical members in the pier with L bent ends that sit over the top of the circle bent piece in the footer to reinforce against bending forces in the column 


For a bridge just built here, they spiral-wrapped the vertical rebar reinforcement with a long helical bent reinforcement as a seismic forces precaution to prevent the rebar from shedding the concrete (rebar doesn't have very good strength against compression and bending).