Why are metal divider strips used in terrazzo floors?

In the construction of terrazzo flooring, metal divider strips are primarily used to control shrinkage cracks.1

When using the traditional method, these strips are arranged in a grid pattern over a layer of cement and sand grout before the mixture of colored cement and marble chips is poured into the sections. Once the material has hardened, the entire surface is machine-polished, which exposes both the decorative marble chips and the metal dividers. In thin-set applications, which typically utilize epoxy resins, the strips are placed directly onto the subfloor before the binder and chips are applied.1