As a nursery/preschool teacher (have done both under 3s, over 3s, and preschools that take 2-4 years)...
It's massively beneficial. I've seen tiny, shy, anxious 2-year-olds who wouldn't even eat what their mum put in their lunchbox change into confident 2.5-year-olds eating everything and interacting with their peers.
I've seen 2-year-olds with speech impediments progress in leaps and bounds due to just interacting with so many other children.
I've seen the shyest little girl make strong friendships and learn how to socialise with others.
I've seen a girl with only rambunctious older brothers make friends with children her own age and learn how to interact appropriately with children her own age.
I've seen a very little boy with teenage sisters find his own friendships and learn how to navigate them.
I've seen a 2-year-old boy with zero English and zero social skills learn how to appropriately interact with children of his own age and older, how to ask for a snack and to go to the toilet, and how to speak basic English as his parents wanted him to.
Nurseries and preschools are beneficial for socialisation, and in the case of preschools, for preparing older children for school. That's far easier if they've been in that setting for some time.