Adrien's answer has a lot of merits. It is indeed a marketing strategy, especially done at this moment (between the two rounds).
It can also be said that this is a French tradition: the President is the President of all French people, designated to lead them all and serve their interest, and not only the supporters of the winning party.
This comes from Charles de Gaulle, the founder of the Vth Republic and the current French constitution (established in 1958), that the French President should be above all parties. Here is an abstract of his Speech of Bayeux
in June 1946, where he exposes his vision of the role of the President (translation is mine):
C’est donc du chef de l’État, placé au-dessus des partis, élu par un collège qui englobe le Parlement, mais beaucoup plus large et composé de manière à faire de lui le président de l’Union française en même temps que celui de la République, que doit procéder le pouvoir exécutif.
It is therefore the head of state, placed above the parties, elected by a college which encompasses the Parliament, but much broader and composed as to make him the President of the French Union, as well as the President of the Republic, from which must emanate the executive power.
As previous recent examples, you can consider:
- Francois Mitterand, first secretary of the Socialist Party until January 24, 1981, before becoming President on May 14, 1981.
- Jacques Chirac was President of the RPR (Rassemblement pour la Republique, Rally for the Republic) until November 4,1994, before being elected President on May 7, 1995.
- Nicolas Sarkozy, president of UMP (Union pour la Majorite Presidentielle -Union for the Presidential Majority- the new name of the RPR from 2002) until May 14, 2007, before becoming President on May 16, 2007.
It should be noted that, as member of the UMP/RPR, Chirac and Sarkozy considered themselves as de Gaulle's political heirs. On the other hand, Mitterand was a strong opponent of de Gaulle.
As for Marine Le Pen, this is probably the right timing to do so. Observers of the politics in France often claim that French people "choose in the first round, and eliminate in the second round". After gathering enough supporters in the first round to stay in the competition, she needs now to convince the voters of the candidates that failed to vote for her.
Her move is a way to give her a more universal stature. In practice, does it has a lot of effect? My personal opinion is that no one cares...