For me, it's guns..... and pocket knives.
My hobby is shooting, reloading, and it used to include hunting as the end game... don't do very much hunting anymore, though. I've carried pocket knives since I was about 10 or so....
I see a particular firearm and think, now THAT would be fun to work up a good load for, and it would be great for pig hunting! (see above
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For those not into guns, this is much like the gathering of tools for every purpose... or, a pair of shoes for every outfit.... and one .22 rimfire handgun is just not enough. I need 5 or 6 of them to fill different niches....
As is with most other hobbies, I could likely get by perfectly well with maybe 4 or 5 total... one for each perceived need, but... it's not about need....
Pocket knives are the same.... I could very likely get by with only one, but why limit myself if I don't have to? Variety being the spice that it is....