To me, the role William is playing in the Singapore state visit is not particularly different to the one he played for the S Korean state visit. Tjis time, he will pick up and accompany the visiting dignitaries from their hotel to the official welcome. On the S Korean visit, he picked up and accompanied the President to the unveiling of a memorial.
How is this evidence? How are they needed for state events. They attend state events, but are really not needed. The Queen, DoE, C & C fulfil those roles adequately.
The PoW and the Duchess of Cornwall fulfil that role. Further, HM and the DoE are also supported by their other children, when required.
Why? They are not full time royals at the moment, so it is perfectly reasonable for them to attend the important events, and those in support of their charities.