One project/editing is good for sim
ple work but if you want to edit a feature movie, a documentary or even a complex short you will need different sequence.
1) To keep different version and get the possibility to mix them ("ok in the new version is more dynamic but the love sequence was better in the first one").
2) To edit sequence separately and to assemble them after. ("ok, let's see if I put the sex sequence before the first kiss")