why is necessary to run react native run

1react-native start is same as npm start and like you said it's to start the 
2react packager/server.
3
4I have found myself in situations where I had to stop and start the server 
5explicitly using npm start.
6
7To start up a project, you use the react-native run-ios and it starts up both 
8the simulator and the server but in cases where the app has been installed and 
9you want to restart the server, you can stop it explicitly in the terminal and
10use react-native start or npm start to start it up again.