Answer :

there could be many different reasons.
farming collectives 
security needs
Because they would have no trade or skill in a new land to find food and try to survive. A group of people are more likely to survive then just one group of people. Plus hundreds of people were normally sent on a ship to the new place to form a colony or they were having problems with the old country and needed to leave to be them. There are many more too. Let me know if you need any