Aside from the obvious reasons to buy Japanese knives I look upon them as a craft indigenous to a culture that I admire. A culture where philosophy, religious and otherwise, is tightly bound to a way of life. I have started a collection of yanagibas made by the highest end makers I can find as a way to document for myself this artistic craft that I suspect will have fewer and fewer practitioners as the years go by. Jon Broida has been exceptionally helpful at pointing me towards makers I would never have found by myself. The stuff that is out there is unbelievable.
For me, it's the fun and enjoyment I get from knowing my tools better and sharpening them better. I'm a home cook, so for me it's really just the enjoyment factor.
As an example, I picked up my usuba today for the first time in a long time. I'm usually too lazy to use it - if I need to quickly turn a cucumber, zucchini or even a daikon into a sheet to do whatever with it, I usually do a katsuramuki on it with whatever I happen to have in my hand. Usually that's either a gyuto or a nakiri. I've been known to do the same with a petty on a few occasions as well. But tonight I was prepping all veggies and decided to dust off the usuba and wow, for the first time ever, it was actually fun to use! Katsuramuki with the usuba was so much easier to do well with the usuba than with the double-bevels (duh!) - and this was just on an edge from my early days of learning to sharpen freehand.
It's the fun from learning and fun from improving that keeps me going. I know there's still much more that I can improve in my technique and knowledge, so that's going to keep me going for a while.
Plus, the personalities here keep things interesting all the while
I'm in the collecting stage, I suppose. I feel like I know what an awesome edge is and how to get there but I'm still constantly experimenting with different finishes, stones, etc. I also have this wierd desire to know more. I see someone compare a knife to something I already have and something inside me compels me to borrow or buy the knife to check it out. Right now, I'm really interested in extreme edge retention (holding a super sharp edge), the effects of differential hardening. I'm also interested in razors and although I've never been into "pretty" knives, there's just something about Salty's feather pattern scimitar... Anyway, I know I have a lot to learn yet, I have made some friends here and it's a place to share some thoughts with like-minded folk and that's not easy to find otherwise.
The variety of knife styles and sizes.