From The Washington Post:

They help members signal to their colleagues and donors that they are ambitious and serious players, said Marian Currinder, a senior fellow with the free-market think tank R Street Institute’s Governance Project who has studied the growth of leadership PACs.

“It seems that the usefulness of them is more of an intra-institutional use, in that it’s still a way for members to demonstrate to party leadership and the steering committees and to their colleagues that they are team players,” Currinder said.