Sometimes the seller needs more time to find or renovate a new property. Or the buyer might want more time to get their ducks in a row for a development project, or they just need time to sort out refinancing etc before settlement day.
I've never needed a long settlement. Never bought from a seller who wanted one, either.
In my personal expereince, many of my deals have presented well to the vendor because I always go unconditional and with the shortest settlement I can, usually 21 days, but 30 if I want some extra leeway.