The short answer is that settling in ethnic communities made immigrants feel comfortable, and they could decide when and to what degree they wanted to participate in the wider culture. So they could immediately do the basics of life - communicate with their neighbors in their own language, rent or buy a place to stay, buy food and other necessities, get the news of the day, and find a job - all without immediately being inundated with language and cultural challenges as soon as they got off the boat.
The disadvantage would be only if they stayed in that condition, and never ventured beyond their ethic community. Their education and job opportunities would be limited, their access to commercial and government services would be restricted, and they might pay higher prices for goods because they were only shopping in the local ethnic markets. They could be victimized by criminals in their community if they could not communicate effectively with law enforcement. They could be victim's of "fake news" if they were only getting their news from TV, radio, and newspapers in their language.