Why does adding steel bars strengthen concrete?

It deals with the tension and compression of the concrete. Concrete is a really great material when in comes to compression. It has a high strength in compression but little in the way of tension. You can test this for your self. Get a small unreinforced piece of concrete and pull it apart it ( put it in tension) and it breaks fairy easily. Now, try pushing together (put it in compression) and it doesn't break anywhere near as easily, if you can break it at all. Steel is the opposite. It has more strength in tension than in compression. Putting steel bars in concrete allows you to have good strength in compression and in tension. In an easy way to understand it and get the general idea, when the reinforced concrete is under tension, the steel bars take over, and when the reinforced concrete is under compression, the concrete takes over.