These are to ensure that your main bus doesn't become unbalanced, as they pull from both sides of the belt evenly.
To use, place a splitter at the edge of your bus as normal, then add one of these instead of a regular belt. (See example blueprint.)
Yes, these are a lot more expensive than using one or two belts and adding occasional lane balancers to your main bus would be cheaper, but these guarantee that you won't find your factory output limited by unbalanced belts at ANY point.
--KFC