Phil, I only provided the answer and dwg because you asked for an explanation of the "why" of a rear toolpost, not as a recommendation of what to do with your lathe.
Although, parting isn't always easy for even a rigid lathe system -- particularly for thin or springy stock, and very particularly for turning crankshaft journals for a model. If you've ever tried to do this from the solid, you know what I'm talking about. And here's another reason:
A lathe tool which is flexible, even when mounted on a heavy rigid base, still has an effective center of bend radius below the center of the turned piece. See the first dwg below. Thus it may chatter or dig in. This is because the top surface of the tool bit is the point of contact on the workpiece, while the support resisting the cutting force is at the tool's lower surface.
Interestingly, the deeper a tool is the worse this situation, and parting tools are often very deep to resist bending. As a result they don't bend, which is good up to a point, on a heavy stiff machine. But on a more flexible small lathe they do dig in and either break or spring the carriage and connections -- possibly causing damage. They are the worst possible shape for that kind of lathe.
A crookneck bit (seldom seen any more) raises the the bend radius above the workpiece center. It hinges above. Therefore the tool bends away from the cut rather than digging in. It was used for older and less stiff lathes, and often had to be made up by blacksmithing.
It just occurred to me that one way to get around the problem of a conventional lathe bit being pointed on the upper surface, yet resting on its lower surface, would be to braze a short length of tool to another, as shown in the second drawing below. This would put the cutting support in line with the cutting edge, and the tool would tend to bend away from the cut rather than dig in. It also would likely be more rigid than a crookneck cutter, being shorter and more in line, and a lot easier to come by. This could definitely be shop made with a torch and solder, and doesn't require a blacksmith.