Why external audio interfaces are better than the onboard sound boards? e.g. The sound recorded from a guitar is better if we plug on an external audio interface (like behinger U-phoria) than onboard sound boards. I would like technical and theoretical answers.
I think the question should be "why do GOOD external interfaces sound better", because I have heard some nasty external interfaces that aren't much of a step up from most onboard systems.
but the short answer is because the external audio interface is purpose-built for sound quality.
Onboard audio units are designed to be cheap and easy to integrate, but not designed for overall audio quality.
The technical differences may seem subtle, but really they are night and day different in how they handle both analog and digital sides of the operation, with a quality external interface taking comparatively much better care of the signal through the whole process.
And if you think the Behringer sounds good, you should hear an RME or Lynx unit!