It doesn’t bond better. Direct dyes, like the salt-moderated component of Rit, aren’t colorfast with or without salt. What the salt does is to get more of the dye molecules onto cellulose fibers, which is why Rit says, “For best color results when dyeing fabrics containing cotton, rayon, ramie, or linen, add 1 cup salt…” (Rayon, though “synthetic,” is actually cellulose). Salt is not necessary in a small container of concentrated dye, only when the dye is dilute, as in a washing machine. Dye molecules are constantly jumping on and off the fibers and associating and disassociating with water molecules. Salt ties up the water molecules so that the dye spends more time with the fibers, and ultimately more dye ends up in the fabric. This is, by the way, the same mechanism that allows salt to thicken shampoos, which is why some manufacturers make a big deal about their “salt-free” formulas.