Delaying solids gives baby greater protection from illness.
Although babies continue to receive many immunities from breastmilk for as long as they nurse, the greatest immunity occurs while a baby is exclusively breastfed. Breastmilk contains50+ known immune factors, and probably many more that are still unknown. One study has shown that babies who were exclusively breastfed for 4+ months had 40% fewer ear infections than breastfed babies whose diets were supplemented with other foods. The probability of respiratory illness occurring at any time during childhood is significantly reduced if the child is fed exclusively breast milk for at least 15 weeks and no solid foods are introduced during this time. (Wilson, 1998) Many other studies have also linked the degree of exclusivity of breastfeeding to enhanced health benefits (see Immune factors in human milk).
This is also another good reason to delay solids even if baby seems to want them. <3