Yes, for us it has been both the range of activities and the socializing. Both of my rampant extroverts have thrived/are thriving on five full days at a large play-based preschool. We did need the childcare, and were very lucky that what worked for the adults was also a good fit for our kids' temperaments.
DS7 knew letters and numbers, colors and shapes before two so needed none of that. For him the value was in a strong group of peers at the same developmental level (physical and cognitive), tons of physical activity (two hours of outdoor play daily plus a very active classroom) and an endless buffet of engaging activities to choose from. His teachers were also brilliant at helping him along with sensory issues, giving him space to escape from loud noises or too much stimulation and giving him freedom from the routine (eg circle time) once it became clear that was what he needed. His preschool friends are still among his closest.
DD3, now in the same class, is much more "workbooky" so chooses to spend more time narrating and writing. Tons of building (three or four different construction projects with different materials going on at any time) and imaginative play (with six or seven kids pulled into different roles as the game expands) that would be hard to reproduce at home. A social laboratory (really, that feels like the way she's using it) of kids on all levels to engage with, plus gentle encouragement to be as comfortable speaking in front of the whole group as she is one-on-one.
The school is Bank Street-based so everything is taught through exploring the world - music, movement, drama, science, read-alouds all integrated into the unit du jour. And it's a large school (25 kids and five teachers in our class, five classes total), so an added benefit is the 15-20 pullout classes - sports, cooking, dance, music - that kids can take (for an extra charge) during the day. We never had to run around to give the kids a range of activities to try.
Honestly I wish the public schools (in the closest three towns, anyway - have preschool friends in all of them) were half as good as our preschool.
