When you have built a Town Hall, you will be offered nomads in your city. To facilitate this proposal, you can create a significant number of jobs, and homes. And probably a trading post.
It will always be a bad idea to accept them unless you no longer have enough children. Indeed, nomads are under educated and far less productive than your current citizens. They will almost consume as much as they produce what can be dramatic in winter. It will be better to be patient and wait until your students become skilled workers.
Also, you have to know that with nomads, there will be more murders and accidents at work.
After a disaster, if the number of children in your city decreased too much, then receiving nomads becomes a necessity. Another case in which you have no choice but to accept them is when you are severely lacking a certain resource, such as firewoods, and you need more loggers or wood cutters.