Interesting discussion! Why do we collect, and why do we collect what we collect?
I can tell you that for me, my hobbies are a fickle muse. I haven't bought a watch in at least 2 years, and in that time I've barely ever lifted the screwdrivers and tweezers to work on them, after about 10-12 years of nearly obsessive collecting. When I was at my peak, I doubt many days passed without at least one watch being en route to me from some Ebay seller, and every Saturday and Sunday, and some evenings, I'd be at the bench making them run again. I found my grail - an Elgin Bumper automatic wristwatch of a grade that I don't think ever made it to market - I took it apart to understand what was different about it and why Elgin didn't market it, despite setting aside several thousand serial numbers and cutting and stamping the plates.
I wrote it up in a blog post while servicing it.
And suddenly I stopped poring over the new Elgin watch listings on Ebay, which I used to go through several times a day. I still wear them, in fact every day I wear both a vintage wristwatch and a pocket watch. But the obsession was gone.
Next thing I knew I was buying and fixing up vintage road bikes and obsessing over those, so now I have a collection of 10 bikes. But I haven't bought a bike in a year, either, after finally buying the Titanium Litespeed I've wanted for almost 30 years.
Collecting, for me - whether watches or bikes - has always been about the history, and about working on them, and about USING them. I've said before that I won't own a watch I won't wear. As far as which watches to collect, I went where my interests went. First it was the last 10 years of Elgin men's wrist watches, especially the Durabalance models. Then that expanded to include essentially all of them after the 8/0 and 15/0 movements were introduced. Then into pocket watches. First the 10s models from after 1942, then the later 16s models, then the early "named" keywinds, and finally just the 571 - each case model and the different transitions. Eventually I achieved my goal of one from the FIRST run and one from the LAST run, and again, once I had that, I lost interest in looking for more. At no point do I remember having made a conscious decision to focus on a particular area - they interested me, so I became obsessed with finding them.
So for me, it's definitely true that its not a rational pastime. Intellectually stimulating, but emotionally driven.