I am extremely lucky to own a buisness with my husband where my kids can stay home with me. However, I do send my now 4 year old to day care part time and have since last year. The reason being, she wasnt getting the interaction with other children that I thought she needed. After being at this in-home day care for a year at 3 years old she can spell and almost write her own name and can count to 20. Not saying its for everyone, but a day care was in my situation was the best decision.
I think interaction with other children their own age, lets them learn and socialize easier. JMO