QUEST RESEARCH - Market, Political, Governmental Research and Analysis Since 1966
Approximately three out of every four members of congress have set up leadership PACs; that’s more than twice as many as existed in 2000.  The surface explanation for these PACs is that they exist to help fund campaigns of others, especially more vulnerable incumbent members of their party, or help fund a promising candidate who may be challenging an opponent of the opposite party.  Since spending restrictions on these PAC funds are not particularly strict, and with the rapid growth in both numbers and funds raised/spent, policing has been difficult with fines for violation merely a minor inconvenience of doing business.  
There are additional, and perhaps more important, reasons members of congress set up PACs, however.  First, almost everyone one is doing it.  Politicians are noted for their desire to be with the crowd.  Second, the PAC allows the politico to double dip so to speak.  That is, politicians are able to raise revenue from the same donors, PACs, etc. who give to their campaigns allowing both the MC and the contributor to get twice the clout for the investment.  Third, by giving to colleagues and friends of their colleagues through the leadership PAC, a congressional go-getter can win enhanced credibility, solidify friendship among those who determine congressional leadership positions, and pave the road to more prestigious or politically advantageous committee assignments and chairmanships.   In addition, the PAC contribution to a local or state party organization for “party building” is a nifty way to demonstrate party loyalty and create a network of friends around the country.
Only about half the money raised by leadership PACs is contributed to other campaigns though.  Some is used to increase personal staff, travel, play golf, court lobbyist-supporters, entertain like royalty, and generally live the good life.   For presidential want-to-bes use funds are a nifty way to woo supporters and line up delegates.
May not be exactly what George Washington Plunkett had in mind when he spoke of honest graft, but that was then and this is now!