SER is a stateless proxy (SIP express Router). SER by itself it not very useful but SER teamed with Asterisk is how you make Asterisk scale. SER normally has nothing to do with the RTP stream. SER only knows and works with the SIP messages that transport information about IP addresses, ports, codecs to be used, etc ... but not with the voice itself and the RTP messages.

SER is a good solution to handle simple stateless SIP message proxying. Asterisk is a stateful proxy and is fully aware of the state of the call and owns also server features that depends of the call state like IVR (Interactive Voice Response) services that must work with the RTP messages. Therefore, Asterisk is most certainly not a stateless proxy server. (Doing an analogy, SER is like an IP router. It knows what's attached to it and what path to send it. It has no idea what is in and what it is sending but it knows how to get it there).

Asterisk offers more protocols than SER. Asterisk can work with SIP, H.323, IAX, Megaco, etc ... whereas SER only work with SIP protocol.

Briefly we can say that SER +ASTERISK is a strong solution because SER does the SIP Proxy functionality being able to manage a lot of connections in a small time, watching for the security and the access control and additional functionalities (for example: to send SMS messages) that not require voice channels. On the other hand, Asterisk manage the voice calls and the connectivity between different telephone networks and protocols (PSTN, VoIP, etc...) and giving the functionalities of a PBX like automatic answer, autoreply, menus, etc ...

One last important thing is that SER can manage NAT for the SIP messages but Asterisk must manage NAT for RTP messages.