Once apart , you will see that the outer surface for the splines is tapered, so as the nut goes up the taper [ tightening the nut] , it closes the outer splines inside circumference size smaller ..thereby tightening on to the inner spline shaft [which has a fixed circumference size].. it is a CLAMPING device employing a wedging force principle. And that force is calculated by the designers to be a specific and exact force for that application. So, they have calculated that at a specific rotational torque on that nuts threads, the wedge design will exert the required force thay want to get the proper backlash specs on the splines that they have decided to be appropriate for that shafts input .
It is specific and handtight does not make it....I have seen plenty of worn splines b/c they were not torqued for proper backlash ..same goes for the rear tranny flange nut. Guys just smack em down with a punch amd hammer [ ouch!!] and a few miles later, the speedo doesn't work [ first bad sign] b/c the clamping force of the tailshaft helix is lost and a little later the flex disc goes , along with the splines on the rear flange, etc.....
Doesn't take any longer to do it right.

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A Dalton