All very good uses for the secondary flight plan.
Other two common ones are for circling approach, where we have the full instrument approach, plus the break off to a downwind position in the primary, and just the actual landing runway in the secondary. Also if in mountainous areas and you have specific oxygen escape routes, then I normally put these in the secondary. In the very unlikely event of a pressurisation failure then there's one less thing to think about in what is going to be a very hectic time.
Basically it's very useful for various what if scenarios, to help take the workload off you when you get a sudden change of plan.