A lead arranger is the investment bank or underwriter firm that facilitates and leads a group of investors in a syndicated loan for major financing.
This firm is paid a fee by the borrower to arrange for the financing. The amount paid will depend upon the complexity of the deal. Risk factors are also taken into account when determining what to charge.
The deal is then doled out to other underwriters. These are the parties that get the money raised by placing the deal with the proper firms.
Under such a scenario the lead arranger is responsible for the deal and getting it funded. When dealing with hundreds of millions of dollars, that is too much risk for most entities. This is why it is spread out.
The arranger is the one who puts to together the syndicate.