Adaptive bitrate selection seems important if your clients have narrow or unstable bandwidth (e.g. mobile), or when you may get constrained by outgoing bandwidth (/. effect aka HN crowd).
imo this is mainly due to the constraint of keeping clientside pre-buffering as low as possible.
sending the whole file at network-speed usually works fine, but maybe uneconomical at scale especially if coupled with users that skip a lot and tend to not watch the whole thing anyway.
