Does it have to be "bring in"? Can't I just use "bring customers"?
"Brings in" is a phrasal verb that in this context, is used to refer to the reason that money is being received or earned. You will find that this expression is commonly used when describing the source of a sum of money. The full expression is "to bring in," and the "in" cannot be omitted. Ex: International students bring in millions of dollars annually for universities in the United States. (International tuition fees account for much of universities' profits) Ex: Our seasonal sale has been bringing in many new customers. (Seasonal sale is the reason for new customers)