Although the technique was developed by opera singers for use in opera, there are singers in a variety of fields that use it successfully. The point of it all is to ensure longevity of the singing voice, and to ensure resonance and tone. In fact, with this technique, nodes are virtually impossible.  The cornerstones to the Bel Canto technique are the "lift of the throat," the "mask of the face," the"inhalation of the voice," and the "hold of the breath."

The genuine Bel Canto technique can't really be taught (actually no technique can, really...) by writing it down and reading it, but Lamperti wrote a number of ideas about the technique around the turn of the century.  One of the cornerstones of the technique involves the "inhalation of the voice." 99.999% of people will, of course, claim this to be impossible, but it works. (more on that later) Using the Bel Canto technique will have one singing stronger as the night wears on, and stronger towards the end of a tour, rather than vice-versa which is a very important consideration for singers. The technique all lies in physiology.... it is an "extension of speaking" and "effortless." (phrases in quotes come from my instructor, who is in turn quoting from Lamperti's Italian, as translated to English)  It should be as relaxed as speaking.  With Bel Canto, there is always a balance - always a mixture of head and chest resonance (never one nor the other exclusively), always a balance between projection and range (never one at the expense of another - they are built simultaneously), etc.

The technique, as I have described and am learning to use, is discouragingly a dying art. Why? The lessons are BORING!! Repetition, repetition, repetition..... 25 minutes of exercises, and 10 minutes of repertoire.  After ten years of lessons, the first 15 minutes are still basically the same exercises I was doing at the beginning.  The results, however, speak for themselves.  According to my instructor, "It will become natural or automatic through large quantities of repetition." The exercises that I do focus on these aspects of the technique in order to make them automatic.

It is hard to teach. Essentially, it is a sensation... hard to explain and conceptualize. You also can't see it, so it is difficult to show. (and impossible to read and write...)  Also, many of the people who used the technique when it was widely popular (the "golden age of singing" around the turn of the century) did not teach it (busy singing, kept it as their secret, whatever....), or at least did not teach it well. I'm sure there are other reasons.

While I agree partly that the Bel Canto technique results in certain tonal characteristics, a large part of your tone is just a product of who you are and how you're built. A Fender guitar will not really sound like a Gibson, no matter what you do to it. Beyond that, it's all a matter of how you approach it. A good example of this is Brian Vollmer. He has been the lead singer for Helix for over 30 years, but also sings at weddings and other functions. If you listen to any Helix recordings, you wouldn't readily guess that he uses an operatic technique. Part of it does involve taking liberties with the technique for "artistic" purposes. Listen to his rock performances, and then listen to him as a classical soloist, and you'd almost never guess it was the same singer!