How to Create a Spreadsheet in a particular folder via App Script

As suggested by @Joshua, it's possible to create a Spreadsheet (in a specific folder) with the Advanced Drive Service (you'll need to activate this if you haven't already, by going into Services +, find Drive API and click Add).

var name = 'your-spreadsheet-name'
var folderId = 'your-folder-id'
var resource = {
  title: name,
  mimeType: MimeType.GOOGLE_SHEETS,
  parents: [{ id: folderId }]
}
var fileJson = Drive.Files.insert(resource)
var fileId = fileJson.id

No need to move files around with this method !

Since you can no longer create Google Docs (Docs or SpreadSheets) using DriveApp, nor use addToFolder because DocList is deprecated. There is only one way to create or "move" Google Docs or Google SpreadSheets..

  //"Move" file to folder-------------------------------//
  var fileID = '12123123213321'
  var folderID = '21321312312'
  var file = DriveApp.getFileById(fileID).getName()
  var folder = DriveApp.getFolderById(folderID)
  var newFile = file.makeCopy(file, folder)

  //Remove file from root folder--------------------------------//
  DriveApp.getFileById(fileID).setTrashed(true)

As you can see this DOES NOT move the file, it makes a copy with the same name in the folder you want and then moves the original file to the trash. I'm pretty sure there is no other way to do this.