G'day Bob...
Nikwax is a leather treatment and waterproofing product. I use it before every walk these days. I used to use
Sno-Seal (beeswax waterproofing treatment), but Nikwax is water based, is easier to apply, makes less mess... and... well... I just like the name.
Nikwax (and other similar products) soak into the leather and make it somewhat more waterproof (although no waterproofing lasts all that long on a wet muddy track). It is also supposed to preserve the leather... prevent it from cracking and deteriorating. With Nikwax, at least, it comes in several varieties, depending on if you like your leather hard and firm, or soft and malleable. Not sure if there is really much difference in the varieties... I've always used the same one.