This question assumes that daycare is just, generically, a warehouse for kids. That is certainly true of some daycares. Some do perform a useful pedagogical function, more in line with what one can expect in more well-developed public state kindergarten/preschool/daycare systems in the Western European states. Moreover, as a sibling commenter pointed out, they get to see other kids there and develop socially--and this is particularly important given the propensity of most Americans to live in fairly insular suburban bubble-houses, and more generally the lack of a public realm and civic space that suburbia entails. It's not like they'd otherwise be out running around with other kids; they'd most likely be marinating at home in front of the TV.

I'd go so far as to say that not sending one's child to some kind of playgroup before school age is irresponsible.