In case when we have two or more SSIDs on one AP, and one of them need to have more care - like VIP network for boss, or open network for guests and private for employees, or we want to sell guaranteed high speed access and leave low speed for free clients - in this case bandwidth shaping for each client is not enough.
10 guest clients with shaper of 2-4 MB/s can waste all resources on AP, and we can't guarantee quality and speed for VIP users. In other hand, when network if free, one client can't get more then his shaper allow.
We can't limit max number of connected clients for AP if it's not many APs in area - some devices has low usage, they may stay connected with no impact on other clients, but those who come later will not allow connect to network.
Bandwidth shaping for VLAN or all clients in certain role on AP can solve this problem - we limit guest or not important SSID and guarantee that payed or VIP SSID will have it's bandwidth on every AP.
This will help us to sell WiFi better as we will offer more valued features.