I am so bad at explaining this but when you take a new diapee/nappy from the pack it is folded with the fold lines creating natural channels outward toward the legs, if you make a counter fold lengthwise it theoretically helps make a channel from front to back.
The channels can be important to direct the wetness across as much fluff pulp as possible until the SAP has time to absorb and swell.
The lengthwise fold also may help fluff the diapee/nappy up so the wetness can move more easily giving you greater utilisation of an individual unit... I find it frustrating when the front is soaked and leaking and the back is completely dry... I am not convinced it is only gravity at play since wicking naturally occurs I believe if the fluff pulp is too compressed (remember they are often tight in the packs) and the SAP swells up at the compressed part it can actually act as a barrier to the moisture moving further kinda like a sandbag.
I first learned about the vertical/counter fold here at ADISC but have since remembered seeing people do something similar with older style baby diapers way back.