Why use senary?

Easier counting

Think about when you have a bunch of objects and you try to count them (like a bunch of coins): you usually group them into groups of five or ten. Then you count up the groups of five or ten to figure out how many objects you have. In senary, you would instead group objects into groups of six. Then you would count up the groups of six to figure out how many objects you have. In my experience, it is easier to make groups of six than groups of five because six objects can make a 2x3 rectangle. Also, it is difficult to visualize a group of ten objects, while a group of six objects is fairly easy to visualize.

It is also possible to count on your fingers to a higher number in senary than in decimal. In decimal, we typically count to ten on our fingers. But in senary, we can count up to fisen-five (or thirty-five, in decimal) quite easily.