Theoretically, we could use all kinds of materials to reinforce concrete. Generally, we use steel because it expands and contracts in the heat and cold roughly as much as concrete itself, which means it won’t crack the concrete that surrounds it as another material might if it expanded more or less. Sometimes other materials are used, however, including various kinds of plastics. (Yes, plastic https://www.sustainabilitymatters.net.au/…/reinforcing-conc…)
If a structural member like a slab is supported from two walls, the slab experiences a lot of compression/being squeezed at the top of it, while it experiences tension/pulling at the bottom. To understand this principle once and for all, one day take two sofas, face them to each other to where your face can hold you on one side and your feet can hold the other side. Now try to suspend yourself. Your midsection, whether a six-pack or a ‘public opinion/pot’ will be severely feel pulled like the forces of your weight want to tear you at the centre but your back is being squeezed together. (We take no responsibility for any injury or embarrassment that may arise after you attempt this experiment.) Steel can withstand pulling, concrete can withstand being squeezed. When they are used as a composite (two materials working as one unified material), then the slab can hold its weight plus added weights and external forces more efficiently.