SOAP is used by ONVIF so if use ONVIF you are using SOAP. And there are likely some manufacturer APIs that use SOAP.
As a user or integrator, SOAP generally won't have much of a practical impact (e.g., it's not as if you can switch to a REST version of ONVIF).
Where did you hear about SOAP? Where do you think this might make a difference? Is any vendor promoting it to you or?
Yes, SOAP is 'already compiled when the camera is already ONVIF compliant.' It's like buying a new car and having the transmission included. It's a fundamental component of ONVIF, not an add on accessory.
Maybe some BMSes use SOAP but even so, it's the same thing, end users and integrators don't need to worry about configuring or setting it up.
I am not sure why Avigilon or Bosch would list SOAP, maybe they just want to list everything to make it look as good as possible but you are not going to need to do anything with SOAP.
Since it is a fact that ONVIF is already using the SOAP I wonder why these two manufacturer still included or stated SOAP on their protocols supported.
In this case of Avigilon, the reason seems to be that they use it for their own propreitery ACC API, since on their 29MP erstwhile megapixel champion, which no one would accuse of being ONVIF compliant, SOAP is listed right after TCP.
But unless you need to do some exotic firewall punches, I agree with John that there is probably no significance in them listing it...