Its a useful feature because modern day browsers do not natively support bandwidth management, they'll utilise as much of the network as possible.
For example to actually stream and watch a youtube video in 1080p a user only needs 733kb/s+ constant download.
However browsers like IE chrome safari etc will max out the users internet connection buffering ahead which will screw over anyone else attempting to use the internet, add in bad router QOS and you end up with packet delay/loss.
Where i live, the internet standard is still ADSL2, (24mbit down 1mbit up) so 1 person watching netflix on the best possible adsl2 connection will eat up all the available bandwidth, this is avoidable by restricting bandwidth on a per user basis, the content (netflix) remains watchable but instead of buffering 10-30seconds ahead it only buffers 2-3seconds ahead and doesnt cause the internet to crap out for everyone using it.
tl;dr Some parts of the world have horrible coverage and by restricting application bandwidth usage you can greatly improve the quality of the connection.
Online Game Distributers like Steam, Origin and hell even torrent clients have inbuilt bandwidth throttling/limitations to deal with maxing out internet connections, imovie/netflix appleTV etc do not because its assumed the person using the service has internet that meets a standard, most of the time it does, but share that internet service and more than often it does not.