So from this point of view its very useful file & manages reusability & allows a user not to import frameworks again & again for each view controllers. Now if in your project prefix file is not present you can create it manually by following simple steps.
Here are the step to create and setup a Prefix Header File for iOS,
Right click on resources folder & click the new file
Now go to targets -> build settings & find prefix header row. Now set a path for prefix header in the row. A path would be like, APPNAME/PREFIX FILENAME
Read more: Healthy iOS Development Tips & Tricks