Learning things like putting their hand up to speak, waiting in line, following a rigid routine, etc are a bit easier when the adult ratios are kinder so six months to a year of a good quality nursery can be a good way to get ready for school. It can also make starting school a bit less scary.
Other than that, unless they are neglected at home, there isn't any need for them to attend nursery.
Some children really enjoy it which is a good reason to send them. Some parents find it helpful to have a break or time to catch up with jobs so they can focus more on the child when they are at home.
Some people, including professionals have the idea that children learn social skills better in a group setting but, in my experience, that isn't true. Most children have lots of opportunities to play with friends and family and they learn those skills then.
Spending the day with an engaged and enthusiastic adult who talking to them about everything they're doing, sharing experiences and helping them explore what interests and excites them is exactly what small children need. They don't need to be immersed in groups of other children their own age.
I was a childminder when both my DCs were coming up to school age so I sent them to pre-school for a couple of days a week for the last two terms and I recommended that the parents I childminded for did the same. It was helpful for them to get to know the children they would be at school with and get used to a more regimented environment. They didn't lose anything by not going when they were younger.