Complex sentence: The main advantage of this system is the total separation of presentation and content: users can concentrate their efforts on structuring and writing the document, rather than concerning themselves with the appearance of the document, which is left to the export system.
Split into simpler sentences: The main advantage of this system is the total separation of presentation and content. Thus, users can concentrate their efforts on writing the document and structuring it rather than concerning themselves with the appearance of the document, because appearance is left to the export system.