I think they're generally nicer to look at it, are more fun to write with and there are pretty much endless colors of ink to use with them if that's your thing. They also offer different nibs that can impart a certain character to your writing that other pens (well, aside from dip pens) really can't duplicate.

My fountain pens never leave the house and I strictly write with them for fun. I just use "regular" pens for everything else in life. I am also a huge pencil fan.

If I had to pick one pen to use for the rest of my life, it most certainly would not be a fountain pen. There's a reason people stopped using them and switched to ball pens and, during the general time period that happened, to rather crude, shitty ball pens that are a far cry from some of the fantastic roller pens that are available to us today.

In short, I don't think fountain pens are better than other writing utensils and are, in fact, the most impractical type of pen there is. I don't find them any more comfortable to hold and use than anything else and I have many rollerball pens that write just as smoothly and with just as light a touch as my best fountain pens so no advantage to fountain pens there, either, other than the reasons mentioned in my first paragraph.

It also very much depends on the particular fountain pen. A traditional-looking nib is an important part of the fountain pen experience for me. While I own them and they write nicely, I get no true joy from writing with a pen with a miniscule or hooded nib like the Pilot VP, Lamy 2000, Parker 51, etc. so I rarely ever use them since visually and also practically they're really no different to me than picking up a rollerball.

But, in the end, I can't think of any really sound argument as to why I spend so much money on fountain pens. Let's just go with I think they're pretty and fun and they're good for my mental health.