Suppose you want to start an ISP (Internet service Provider).
You will need a group of servers, for example for DNS, e-mail, etc.
You will need to put a router to connect your servers and users to the rest of the world, so the best place is an Internet exchange point (IX). The primary purpose of an IX is to allow networks to interconnect directly (as they are inside the same building), eliminating the cost of installing costly cables from your ISP to high-tier ISPs.
You put your equipments on the IX premises, get agreements with other ISPs on the same IX and now your ISP is public and working. You pay a rent for using the space and electrical power of the IX.
The more connections you have, the better service you give. So, there comes a time when your ISP needs to connect to other ISP not available through the IX, let's call it ABC.
Then you can rent the use of a limited bandwith on a link to ABC belonging to a third-party provider, let's call it XYZ.
Your ISP is growing and then there is a moment that you can consider stop paying a rent for the limited bandwith to XYZ and then you deploy your own connection with more bandwith directly to ABC eliminating the monthly fee.
The ISP keeps growing, so you are considering to connect directly to ISPs on other continents, then you have to pay a monthly fee for bandwith on a submarine cable, unless your business has grown so much that you can deploy your own one.