Traffic inside your network come free of cost of course except for your own bandwidth constraints, but once you need to communicate with other networks that's w

Traffic inside your network come free of cost of course except for your own bandwidth constraints, but once you need to communicate with other networks that's where you start to pay. Google offers free interconnections, but sometimes you can't just connect directly to Google and have to go through backbones that are owned by private companies. So ISPs need to create as many interconnections as possible with other networks to decrease their costs, invest in their own infrastructure and pay for everything their network can't reach directly.