Most mobile apps work on client-server architecture. They need to fetch data from the server and then display it to the user. If you have a diverse range of users, you would need to definitely test their network conditions. Some users will be using your app in very low network conditions. Thankfully, apple provides us with tools to test low network conditions.
Go to https://developer.apple.com/download/more/?q=Additional%20Tools and download the Xcode 11 tools.
In the downloaded package, go to Hardware, and click on Network Link Conditioner.prefPane. This will install the conditioner. You can access this via the System Preferences. This can be used to test network conditions on the simulator.
There are options to test 3G, DSL, LTE, Wi-Fi and so on.
On the device, if you would like to test network conditions, you can go to Developer options. I have attached the screen shots.