Just what the title states, please.
Iron reinforcement bars used to provide the framework over which concrete solidifies always appear to bear grooves. Do the grooves behave as the thread of a screw and provide better grip? Are the grooves merely a manufacturing artifact? Are the grooves a historical artifact - the first bars had to be manufactured grooved for some reason, and hence they continue to be so produced without rhyme or reason.
Why are rebar grooved?