It ultimately, eliminates the flex in your material. This is especialy
significant, in thin stock. Your rollers on your planer table might be a
few of thou above the table itself. As the knives come in contact with your
stock, the stock is thin enough to flex slightly, your more likely to get
tearout, etc,etc. The rollers usually do not present a problem in thicker
stock (the stock won't flex).
Carriage/sled, potato, patato, all you need is something to give you a
reliable/repeatable process. "A jig for planing thin stock".
The longer the better, but even a few feet will do, especially if you don't
plan on using it very often, and your tolerances aren't exceptional.