In France: to avoid split governments
Since 2001, both Parlement and President are elected every 5 years. The electoral law has been organized in such a way that the Presidential Election predates the Legislastive Election by a few weeks.
The aim of this calendar is to allow the President to get a majority in the Assemblée Nationale that will allow him to develop the policies he favors during five years.
Le double quinquennat, mais aussi la liaison des deux élections, dans un ordre respectueux de la primauté présidentielle, ont ainsi, volens nolens, jusqu’à présent, atteint le but poursuivi : inscrire les deux élections, présidentielle et législatives, dans un rythme quinquennal dégageant au profit du chef de l’État une période de cinq ans pour gouverner, avec l’appui d’une majorité au palais Bourbon, plus ou moins homogène cependant selon les législatures.
Linking the two elections with priority for the Presidential one has until today reached its aim: settling the two elections, presidential and legislative, in a quinquennal rhythm ensuring in favor of the head of State a five years period to govern, with the support of a majority in Palais Bourbon (equivalent of Capitole - Evargalo), more or less homogeneous however depending on legislatures.
Indeed, in every election since 2001 (2002, 2007, 2012, 2017), electors have sent a majority of deputies from the party of the newly elected President (Chirac, Sarkozy, Hollande, Macron) to Palais Bourbon. While this outcome is of course not granted by law, the boost the candidates receive from their side's success in the Presidential election has always proven overwhelming.
If elections happened on the same day, there would be much more chance for the outcomes to differ in the two elections, leading to what we called cohabitation - i.e. a split government.
A side effect is the reduced power of the Parlement, sometimes described as been reduced to a mere registration chamber.