Hallo Martin,
it is not recommended and from my point of view not needed to build aggregates for an BW on HANA. If you have no aggregates, you need no change run.
But not everybody has a fully optimized BW on HANA - yet. Companies might keep their aggregates during the migration to HANA database. Or your BI CC want's to keep aggregates for whatever reason. If you still have aggregates while you are on BW on HANA, you still need a change run.
That's why the document says 'may not be needed'.
Will the change run be faster on HANA. I would expect, but I have no comparison values.
Hope that helps,
Matthias