Creating subnets means dividing a large network into smaller ones. These subnets can be created for several reasons. For example, creating and using subnets can help reduce congestion by making sure that the traffic destined for a subnet stays in that subnet. This helps in efficiently routing the traffic coming to the network that reduces the network’s load.

https://www.cloudflare.com/learning/network-layer/what-is-a-subnet/#:~:text=Subnets%20make%20networks%20more%20efficient,routers%20to%20reach%20its%20destination.