Update in the light of Swift Package Manager: If you're building a project as a Swift package - you should definitely ignore this file as it can be generated using file system as source of truth. You can do that by using the following command:

$ cd ~/Projects/MyProjectFolder/
$ swift package generate-xcodeproj

For non-SwiftPM answer - see below.

This file holds the list of all the files in the project, settings of targets and which files belong to which targets. It's probably the meatiest file in project bundle. You should not ignore this file. There are few points for this:

  1. You may not want to work on this project alone or;
  2. You're planning on working on project from different machines;
  3. You'll want to share your code base with others;