The following are the main considerations for prioritizing traffic:

Is there congestion in the network? If not, there is no need to prioritize traffic.
Delay-sensitive traffic such as voice over IP (VoIP) and videoconferencing are more sensitive to delay and hence need a higher priority than FTP traffic.
WAN links with speeds of T1/E1 or lower can benefit from prioritization.
If a WAN is constantly congested, prioritization might not solve the problem. Additional bandwidth needs to be added.