Instagram wants users to be on the app often and build a large following/follow a lot of people. To influence people to do this, it has suggestions for who to follow, and sends notifications to remind you to check Instagram. These pop-up notifications act as prompts for the user to open the app, and because Instagram is an app on phones, people have a high ability to check the app often. It is also quick to load and use, and free, so the app increases ability to check it quickly, when doing routine activities such as waiting in line.