A good scenario is to make sure that torrent clients aren't getting priority (while ideally your enterprise network qos policies should overcome this problem as well)
A bad scenario is overriding DSCP markings from Jabber Client which marks packets genuinely for seperating audio and video streams treatment.
While you can still overcome the problem of Jabber Client using network QoS policies, you can allow QoS marking on windows OS as follow:
1. Go to HKLM\System\CurrentControlSet\Services\Tcpip\QoS. If "QoS" folder doesn't exist there - create it.
2. Add a DWORD parameter named "Do not use NLA" and assign "1" as its value.