I would say this is just one particular way of looking at the process of liberation, that is, by focusing on samatha. None of these attainments is actually required, apart from the ending of the corruptions (the āsavās) at the very end. Yet going though the immaterial attainments seems to be a valid way of reaching the end of the path. Somewhere along the way you are likely to become a stream-enterer, but the particular point at which this happens will depend on the nature of your spiritual qualities. The combination of stream-entry and taking samatha to its peak will then enable you to attain the end of perception and feeling (saññā-vedayita-nirodha) and also the end of the āsavas.
It is interesting that the end of perception and feeling almost invariably is paired with the end of the āsavas in the suttas. You get the impression that seeing everything cease is a very powerful foundation for insight, as one would expect. Yet despite this strong correlation between the two, it does not seem to be absolute. There is at least one sutta (AN 5.166) where the end of perception of feeling does not immediately lead to the end of the āsavas, but instead you go on to another rebirth. But this sutta is really just minor variation. Overall it is clear that the end of perception and feeling is a very profound attainment that will normally result in the deepest possible insight.
So no, I don’t think it is there just to support the description of the Buddha’s final passing away in DN 16.