A good eyeletted box section rim will distribute spoke tension through both sides of the section rather than visible face taking all the strain. On a u section rim an eyelet should help distribute the spoke tension more evenly around the hole, the force being offset in crossed-spoke rims because the spokes do not exit the rim perpendicular. Its worth stressing 'a good rim' because i have seen an eyelet rim pull a spoke through and on inspection found that the eyelet is on the outer section and the inner section holes were formed of ragged metal punched through rather than drilled.