Please find the below explanation, kindly correct me if I am wrong:
Here route tables are used to route the table is a set of rules used to determine the network in which it has to be directed.
Each VPC has its own routing table to send/receive traffic.
Where internet gateway is used to connect the internal IP addresses to the internet or vice-versa.
In general, routing table routes the traffic internally in the AWS network but for the IGW it is to contact the outside network basically internet.