As you know, almost all fountain pen nibs (sans some specialty ones) come with some tipping. This is a very hard alloy engineered to resist wear and tear.
Some manufacturers have their own formulations and grinding characteristics, and some manufacturers use "default" versions supplied by the nib vendor.
As the user writes, this tipping material starts to get polished. This can take from a couple of months (e.g. Lamy) to years (e.g. Pilot, Sailor). Since the user keeps the pen at a certain angle, the same area gets polished a lot.
This makes the pen write smoother when held "correctly" (i.e. the way the user holds), reduces contact pressure (pen starts writing almost before touching the paper) and makes the pen a little wetter in some cases, making it more reliable and enjoyable to use.
After some point you can write without ever thinking about the pen, because it never skips (even like gels, rollerballs and ballpoints), and becomes an extension of you. It's hard to precisely and accurately describe though.
For example, I have an old Lamy Safari which writes slightly broader than its Medium designation because of this. I can understand whether my Pilot Metropolitan is happy with the ink or not from how it feels on paper. I have another Pilot which feels like glass on paper due to the same effect (it was already a smooth grind but it got even smoother over time).
Another advantage of fountain pens is the writing characteristics is a constant. Since you don't change the nib with every refill, you don't get the frustration of a bad writing pen when you replace your disposable pen or refill. You only refill the ink.