If my application can do the work for a VoIP call established via HTTP, why should I use SIP?
SIP actually uses the HTTP headers. I assume that you are using a softphone? There are lots of advantages to SIP including URL dialing, better session routing for all packets, etc. It may be that you are using a program built on this great technology.
But in short, the answer is if you are happy with what you have, enjoy. If you want some additional functionality...
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and some really great add-on applications, I suggest you check out SIP. You may not know what you are missing.