-
Maybe this image helps a bit, offset rim helps to make the hub-rim-spoke triangle more even and allows for more even tension between the drive and the non-drive side of the wheel, which makes a stronger wheel. As @Emyr pointed out a longer freehub body results into a less even tension than a 'normal' shorter freehub body. An offset rim helps to compensate
tldr left triangle is more 'even' and therefore stronger
You never really need offset rims, it just builds a stronger wheel (if that's what you're asking?)