My first firearms purchases were for home and self-defense.
Then I picked up a good (sporterized 1903 Springfield) rifle in .30-06 for hunting.
The a couple more defense pistols and a S&W M29 I could NOT pass up. ($300 for NIB during the 'Dirty Harry' mania of the middle '70s.)
Then I bought some specific target type pistols. Bullseye at first. Then something(s) for IPSC style shooting. Then a PPC revolver.
Since I was a lawman, I started picking up 'cop guns'; mostly revolvers, but some autopistols of the 20th Century. And some 'significant development' handguns, like a Browning High Power and a S&W M39. Lately I've been collecting .32 Automatics of the Art Deco period. Three Savages, a Colt, a Harrington & Richardson, an MAB, a Beretta and so on. And I just like Ruger Single Actions (three screw versions) in .357 and .44 Magnum. And Government marked .45 Awfulmatics.
I've even got a couple shotguns for bird hunting - provided I can hit the silly things.
There's some order and plan, and some "That's neat!" factor. But I still have places to put them, so I don't have too many yet. Nor too many books; same rules apply.