If a company issues 1000 shares @ $10 each for public subscription. The company was able to collect $9000. Underwriters help the company by buying the left shares (company then collected 90% of the issued amount). The underwriters get commission for this and they become the shareholders of the company. Will they get any other benefit than commission they recieved? Will they get any more advantages than the people who have bought the shares? I mean, will they get more profit than public who have bought shares?
Underwriters often require blocks of friends and family shares as part of the underwriting deal. They can offer these shares for sale to family and friends (typically important customers unrelated to the firm making the IPO) before the IPO actually takes place, enabling the "friends" to make quick profits when public trading starts. Famously Google refused to do this in their IPO and made some investment banks very unhappy.