Ivory is what much "white" silk and lace actually is. Undyed silk is not pure white, and bridal designers often prefer the softer tone because it looks warmer next to skin than bleached white. Queen Victoria's 1840 wedding gown, the one that set the fashion for white weddings, was ivory silk satin trimmed with Honiton lace.
In lingerie, ivory is mostly a bridal color: corsets, garter belts, chemises and lace bodysuits sold to sit under ivory gowns without a visible color jump.
The classifier draws a narrow line here. Ivory is warmer than white but still pale and without the yellow of cream. In poor light, ivory and white are often impossible to separate.