Very low friction, super-quick to scribble stuff down, easily erased etc. Even the cheapo mechanical pencils are pretty decent IME
I can't say erasure is a typical ability in fountain pen inks. In fact if you look into Noodler's Inks (two of the rollerballs mentioned come from Noodler's Inks) you'll notice that the creator of those inks leans the other way, toward forgery-proof, water-proof, tamper-evident, archival-quality, fade-resistant inks. Use those, and you'd be able to prove that you wrote it, and no one would be able to alter it.
That said, Noodler's provides "waterase" inks for use on material like whiteboards. Wipe the text away with a wet cloth.
I've recently taken to using a TWSBI Eco (fine) at work for note taking. I write maybe two to three pages a day. What amazed me about it is that the nib felt like using a pencil! It is tuned to run smoothly across the page, but provide a very subtle feedback to give it that feel.
By volume, it is probably the majority of ink sold.
Several countries require children to use fountain pens at school, or still have widespreaf use even if it's no longer officially required. Germany, the UK, I think India.
The blue ink that everyone uses most of the time is erasable with a chemical eraser pen [1], which everyone in my class (in the UK) owned.
Other colours were not erasable, so my gothic rebellion (requiring me to write in black or red) meant I had to be correct first time.
[1] https://en.m.wikipedia.org/wiki/Ink_eraser § Chemical
The thing with the erasers was once you erased, the chemical was still on the paper so you could not re-write ont he erased parts. This meant you had to use the other-end of the eraser pen where there was a special blue ink pen. Often this would then "bleed" heavily (perhaps I didn't wait for the paper to dry?) and never looked the same as the fountain pen ink, so it was hugely obvious where you made a mistake as your normal wiring all looked normal, then you'd get this huge blurry fuzzy blue mess where you "erased" a mistake :-)
Now I'm curious. Why is that? How are fountain pens different from regular pens when it comes to school? I've never even seen a fountain pen, let alone written with one (Canada).
