A couple of posters touched on the need (or advantage) of using techniques to help projection and avoid injury. Singing is an athletic activity which uses various muscles. Whether you're self-taught or take some sort of lessons or coaching, every singer "learns" to sing. It is a skill and activity beyond mere talking. How you learn, and what you learn, will affect how you sound.
Some techniques and singing styles were developed for particular purposes. For example, I believe "operatic" singing includes vibrato in part to help differentiate the singer's voice from the instruments in the orchestra. I also suspect that some sea chanteys are to be sung in a very rhythmic way to help the crew hear the beat & work together. Then there are ornaments that might be unique to various countries/regions/time periods (I can't think of vocal examples, but the "snap" of Scottish fiddle music is one such).
When a person is learning to sing a song from a particular culture, is it not appropriate to at least consider using the stylings or ornamentation of that culture? I'm not talking about "accent" per se, but melodic interpretation.
Of course, you can have great fun taking a tune from one culture and applying the stylings from another - such as adding an Irish lilt to "Die Lorelei".
By the way, if you want an example of someone who intentionally chose to sing in an unornamented style, listen to early Suzanne Vega - esp. "My Name Is Luka". She could/can sing in different styles, but decided a bare-bones unadorned voice to separate herself from the "pop" crowd. Is that a "folk voice"? Or is it not, because it's not a "folk song" and the vocal styling was a decision rather than an accident?