This is just what it sounds like - a trailer for hauling your Headers around the world. When you have many fields that are distant from each other and you want to grow a variety of crops at each depending on the time of the year or your whims, you will want to be able to transport your headers - which you really can not do while they are attached to the harvester for obvious reasons...
That said, you can select from the following - or you could pre-position the different Headers you need at each of the satellite farms, so you would only need one of these trailers for the first transport from the Vehicle Store to the satellite farm.
Note: You may be tempted to say to heck with it and try to transport your Headers while they are connected to the harvester. Good luck with that mates - all things being equal we suggest not...