Data center traffic flows in directions.
North-south traffic moves between servers and external clients. Users outside access applications inside. Firewalls and load balancers handle this.
East-west traffic moves between servers inside the data center. Modern applications distribute work across many servers. A single request might trigger internal API calls.
In modern distributed systems, east-west is often % or more of total traffic. If your network cannot handle efficient east-west communication, applications suffer latency.