Traditional scholarship's divides prophet Muhammad's life into makkan and madinahn phases. This is chronologically valid and represents the two broad aspects of his life before and after the watershed event of the migration.
It is historically important and marks the beginning of the Islamic calendar. Additionally i believe muhammad's struggle can be naturally divided into seven phases. Each phase brings forth a different aspect of his personality and highlights a different facet of his mission.
Studying the messenger's mission for its various phases and analyzing its internal dynamics is important as it gives the narrative relevance for today. since his life is better documented than the lives of other Prophets and leaders of major world religions, it is possible to build this analysis on a historical foundation.