Eh... mostly bundles/complete editions, almost always because of a sale ("it was cheap" is able to be folded into that, really). Next notch down is probably due to mods, if that first isn't applicable. There being something interesting fan made that requires a DLC is one of the easier ways to talk me into getting it. Though that said, most of the times I've done that were about as applicable to the base game itself, never mind the DLC. Civ 4 springs easily to mind, there -- I honestly kinda' straight up hate playing the base civ games (I... don't think I've ever actually started a vanilla Civ 4 game, despite probably having at least a couple of days of game play in raw hours), but stuff like Masters of Mana can get me to sink some hours into the thing, and when I got my hands on it, it was specifically for stuff that needed DLC to run.
... last is appreciation. Not because I don't appreciate stuff or want to give them money, but just because the way I play/purchase -- my comps have been consistently shitty enough I'm usually at least three or four years behind current releases. Usually by the time I get to the point I can actually play something, development's stopped and I'm getting the thing in a bundle with all the DLC I care about
Though yeah, that also pretty comfortably nixes season passes. Never had one, probably never will. By the time I get around to something, the pass usually ain't doing anything anymore, heh.
So far as the completionist thing goes, eh. It's not my cup of tea (never mind games, I've started book series two or three books in often enough it's not surprising), but I have family that collects memorabilia so it's not like the mindset's foreign or somethin'. Just not somethin' you'd get me to go out of my way to spend money on.