When a company first enters the cloud, one VPC suffices. But soon they separate dev/stage/prod, create VPCs per team, acquire companies with their own VPCs, and connect on-premises datacenters. At some point, operators must decide how 5, 20, 50 VPCs communicate. The tools they meet then are Transit Gateway, VPN, Direct Connect, Route 53.
The difficulty with these tools is that without understanding what trade-off each solves, you end up with wrong combinations. Connecting 20 VPCs with VPC Peering requires 190 connections; running operations on a single Direct Connect circuit and facing downtime from circuit failure happens somewhere every year