While I like the different answers let me try this a bit more simply:
Concrete is very strong in compression. Such as placing a block of it on the ground and setting a large weight on it.
Concrete is comparatively weak in tension. Treat it like a rope or chain and breaks easily.
Bending, which is just both compression and tension within an element, happens in most cases because almost nothing is in pure compression. So, we add something that is really good at tension: steel, thereby creating reinforced concrete.
The concrete deals with the compression and the steel deals with the tension. In designed members, we can tell where we are expecting to get tension forces and correspondingly place the steel along those points.
MKDesign 