However, they do assist the spoke nipple ‘move’ in it’s recess, and thus reduce the stresses at that point (all 24 of them in your case) of the rim. I suppose they also add some rigidity by tying together the section of the rim, though the metallurgists/engineers here would be able to detail the exact +/- of this.
No you don’t need them. If you build the wheel with eyelet-free rims it’s not bad practice to check and debur the spoke holes on the rim and slip a bit of grease or anti-seize on the spoke holes so your nipples will turn nicely and (hopefully) not seize later. Deburring also helps to avoid stress risers at the hole, and obviously isn’t necessary with eyeletted rims.
Shit…um…they were invented for a good reason, they look pretty when they’re shinny and new, they redistribute stress on the rim at the nipple interface, they’re good enough for the pro’s
Take your pic haha
Ok, how about because double eyelets help distribute the load over the upper and lower surfaces of the rim, which was probably more important in the days of lower strength alloys as compared to the heat treated 6000/7000 series that we use now.