Quality of Service (QoS) is primarily applied to Internet
telephony (IP telephony, VoIP).
In VoIP (Voice over Internet Protocol) telephony the cable
frequently shared with internet connection.
If internet connection uses/occupies too much of cable's
transmission capability by downloading or uploading files, there
will be not enough cables resources to transmit telephony signal
effectively thus making conversation impossible or with low hearing
rate.
QoS reserves certain amount of cable's capability only for
telephony, thus no matter how much internet connection is used,
there is always enough room for plenty of simultaneous telephone
conversations.