The first inkless pens were basic metal styluses used by scribes in the Middle Ages to draw on a specially treated paper surface, usually a form of vellum or parchment. This technique, called metalpoint, was especially popular before the invention of graphite. Since the metal a scribe's stylus was made from was considered a status symbol for his employer, it was common for the scribes of upper class citizens to use a pen made from gold or silver. For the lower classes, lead was an inexpensive alternative.
Several famous artists from the Medieval period used metalpoint, and thus inkless metal pens, to produce at least a portion of their work. Leonardo da Vinci used metalpoint for many of his study pieces. One example is "Studies for the Nativity," which he produced somewhere between 1480 and 1485. Dürer, Rembrandt and Raphael also made use of the metalpoint technique in their work. Rembrandt employed a variation of metalpoint called silverpoint, so named because it requires the artist to use a silver stylus.
There are several benefits to using an inkless pen as opposed to an ink pen or a pencil, most of which relate to its lack of ink. For example, an inkless pen never needs refilling, and will never dry out or clog the way a ballpoint pen can. No ink also means that it can never leak. Inkless pens are an economical alternative to ink-based writing implements. According to Grand Illusions (a company that produces metal pens) the solid metal nib can take more than 25 years to wear down completely.
Inkless pens are simple to care for and maintain. Unlike ink pens, they do not need protection from heat or cold (except for extreme temperatures). Also, they are more resistant to damage than a normal pen because they have fewer breakable parts. The only real maintenance they require is an occasional sharpening. As the Grand Illusions website notes, this is done by filing the nib back to a point with sandpaper when it becomes too worn down from continual use.
The metal styluses used by scribes in the Middle Ages are actually to blame for the misnomer "lead" being applied to the graphite inside pencil shafts. Lead was a cheap alternative to more expensive metals and its softness made it much easier to write with, especially if the scribe in question was trying to write on untreated paper. When pencils were first marketed in the 19th century, the graphite tips appeared so similar to the traditional lead tips that the term stuck.