Policers for incoming network traffic are ideally placed at the edge of the network to prevent bandwidth being wasted throughout the core of the network.
Policers for outgoing network traffic are again optimally deployed at the edge of the network or core-facing interfaces on network edge devices.
There is a downside to policing that it forces TCP to send TCP retransmissions when it drops traffic.
Shapers are typically used for outgoing network traffic and are typically deployed by enterprise networks on service provider facing interfaces. Shaping can be useful where the service provider are policing incoming traffic or where service providers do not police traffic but do limit their service level agreement to a certain amount of bandwidth.
Shaping traffic causes traffic to be queued and delayed, rather than outright dropping it like policing does.