Think of a group that you may belong to; a fraternity, sorority, political group, social club. Now suppose the members of this group never came together and most of them didn't even know each other. All business was done through mail. How much do you think this group would mean to its members? How much enthusiasm would they have for it? How much good would the group really do them?
Likewise, Christians who have little to do with each other and never come together in a group can accomplish nothing. If the early Christians would not have assembled for mutual encouragement and common worship, Christianity would have never survived and we would not know Christ today.