Asterisk vs OpenSER

I have an asterisk box already working, why do I need OpenSER?
Is it faster?
Is it more stable?
It has more hardware options?
Or is it easier?

OpenSER is a SIP server, and Asterisk is a PBX. They are two entirely different applications, but can (and often do) work together to provide a scalable and powerful SIP network infrastructure

Isn't asterisk a sip server?

Yes it is.

But:

Asterisk is a feature rich jack of all trades, it can do a lot of everything, providing all the facilities one expects from a PBX. And that is where the trade off lays, it is not necessarily good at any of them.

OpenSER is a pure SIP Server. As a backbone system it can connect tens of thousands of SIP clients together at once. Asterisk will collapse if you try and connect more than a few hundred users.

Asterisk can convert a SIP call to (say) an analogue telephone, voice mail or a cocoa tin at the end of a bit of string. OpenSER requires external help e.g. Asterisk.

Thank you all very much.
I managed to have a slight opinion about OpenSER.