Both queuing and scheduling are used to decrease traffic congestion. It is recommended to apply the scheduling algorithm only when there is no traffic transmitted / received on that port.
By default, strict-priority is configured as scheduling algorithm (as default scheduler). A disadvantage of this algorithm is that if there is enough traffic in the strict priority queue, other traffic can be completely starved of bandwidth.
Weighted Round Robin (WRR) is one of the scheduling algorithms used by the device. In WRR, there is a number of queues and to every queue is assigned weight (w). In a classical WRR, the scheduler cycles over the queues, and when a queue with weight w is visited, the scheduler can send consequently a burst of up to w packets. This works well for packets with the same size.
In the more general case of IP networks with variable size packets, the part of the bandwidth received by each queue depends not only on the weights but also of the packets sizes. To approximate the generalized processor sharing (GPS), the weight factors must be adjusted based on the packet size. That requires estimation of the average packet size, which makes a good GPS approximation hard to achieve with WRR.
Deficit Round Robin is a later variation of WRR that achieves better GPS approximation without knowing the mean packet size of each connection in advance.