Most people probably won’t need this, unless you encountered such scenario:
You're handling a huge project, and you somehow need to move the entire project folder from a Windows machine to a Mac for porting the project to iOS.
What will happen is when you open the project for the first time on Mac, Unity will load all the files and convert them into Windows formats because well, your project was set to run on Windows previously.
You waited for half an hour or so for all the files to be converted (it' a huge project!) and now you must change the build settings again, and wait for another half an hour for it to be converted to iOS!
If you want to avoid running the conversion twice, you can change your Unity project’s build target before opening the editor, by using the command prompt, like so:
"C:\Program Files(x86)\Unity\Editor\Unity.exe" -buildTarget
Possible options are: win32, win64, osx, osxintel64, osxuniversal, linux, linux64, linuxuniversal, ios, android, web, webstreamed, webgl, xbox360, xboxone, ps3, ps4, psp2, wsa, wp8, bb10, tizen, samsungtv
"C:\Program Files (x86)\Unity\Editor\Unity.exe" -buildTarget ios
This way, Unity will straight away convert your project files to iOS formats and no need to convert it twice. Time saved!