What is a reason a network administrator would segment a network with a switch?

A switch creates microsegmentation, which in turns isolates unicast traffic between two talking computers from other computers that are not part of the communications. This, in turn, increases bandwidth for the computers that are not part of the communications between the two talking computers. The creation of broadcast domains can only be achieved with the addition of VLANs and a router. The isolation of ARP messages can only be achieved by the creation of broadcast domains. Segmentation with a switch will create more collision domains, not fewer collision domains.