Check out Dr Joe Vitolo's "Demystifying the Oblique Penholder" on IAMPETH. He explains it much better than I can. Suffice to say that I disagree with the above reasoning. For the right-handed it's uncomfortable and extremely difficult to write attractive traditional Copperplate with the nib pointed at the 55 degrees angle with a straight penholder. Rotating the paper helps a bit but the tines twist and the shading suffers accordingly. In traditional Roundhand, it was written with a stiff quill cut to a very narrow edge. Hairlines were produced with the corner of the edge of the quill tip. This practice was discontinued with the later invention of the flexible metal nib. The oblique holder makes life much easier with attractive lettering and no hand fatigue.
This was (of course) written with an oblique holder and a conventional tripod pen hold.
