Foundation is important to a building. Infrastructure is important to a network.
You have your internet connection, you have your computer. You then connects your computer to the internet modem provided to you by your ISP.
You are happy browsing, and, one more computer arrive and needed the internet as well, what are you going to do?
Now you have a router connecting both of your computers to the internet and you need to print. You don't want to spend the money to buy two printer, what are you going to do next?