In my opinion, open primaries increase party power by giving the people a voice to change the parties platform and redefine the will of the people through the primary process. Without the ability to change the nature of the party through the open primary process the party and its politics stagnates.
Open primaries allows voters that are not affiliated with the major parties to participate in selecting a one of the major party's presidential candidate. By allowing persons that are not affiliated with either of the major parties to participate in the selection, the major parties lose a least some of their power. This also gives independents a voice in choosing a candidate with the potential to win the election.