The working principle of a fountain pen is related to a complex balance of different forces that make possible that just the light pressure of nib on the paper can make the ink reach the sheet from the reservoir, without leaking when the pen is not used.
A fountain pen should not be used in vertical position, but the nib should be put on the paper so that the lower tip of the nib smears on it. The great advantage of a fountain pen over a ballpoint pen or a roller (that must be used vertically) is that this kind of use allows you to write using very different writing angles, while taking an hand position that is much more natural (you just let the pen go between thumb and index). In addition, the fingers (held between thumb and index, using the medium as an additional point of support) must be used to move the pen when writing, and not to hold it.
Finally, a properly functioning fountain pen does not require any pressure to write, the weight of the pen must be sufficient to start writing. The use of pressure can be used instead to open more or less (depending on its flexibility) the tines of the nib, allowing you to vary the tract size, and thus having a writing result that is a lot more personal.
All these characteristics make writing with a fountain pen much more convenient and comfortable than with any other type of pen, allowing much longer and less laborious writing sessions. For these reasons, even today, despite the presence of so many alternative technologies, the fountain pen is considered by people that do a lot of writing (writers, amateurs, enthusiasts) as the best tool to write and draw marks on a paper sheet.