OpenSER v. Asterisk -- or is there much of a difference?

Maybe someone can clarify a question for me. What can OpenSER do that Asterisk can’t? Or what can it do better than Asterisk? I’ve looked through the docs at to get an idea, but I’m still a little unclear. Any help?

OpenSER is specialized as a SIP Proxy. Asterisk is specialized as a softswitch/PBX. For certain applications they are perfect mates.