My reasons are threefold:
1. I just really like minis. I've always enjoyed collecting things, and I was collecting D&D minis long before I actually started looking for a regular game. But my minis aren't doing anyone any good just hiding in a box.
2. My mind wanders. During the Tantris campaign, I had to give the DM my full concentration every second, which is hard for me. Sometimes he'd say something that would spark an idea in my head. I'd suddenly think, "Oooh! That gives me an idea for a story!" And I'd start writing that story in my head. When I finally returned to the present, I had missed three paragraphs of exposition. It was even worse in combat. Our DM was great at describing action scenes, but it's hard to keep an entire battlefield in my head. I'd constantly lose track of who was standing where. With minis, one glance at the board and you know where everyone is standing, who are affected by which status effects, and even which enemies have taken the most damage.
3. Breaks. When everything is in everybody's heads, then we all have to hear every word to know what's going on. If somebody has to go to the bathroom, then the game has to stop. Otherwise you'll just have to catch them up when they get back, and what's the point of saying something the first time if you know you're going to have to repeat it in a minute. With 4e, you just wait until combat, then get up right after your turn. Someone else can look at your character sheet to help determine if an attack hits you while you're gone. By the time you get back, it might be your turn again, but you can plainly see where everyone is standing now, which creatures are bloodied, and who is dazed/prone/blinded/etc. Nobody has to repeat anything.
Some people think using minis limits roleplay. I disagree with this totally. For starters, out-of-combat roleplay is unchanged. In combat, well, admittedly 4e's combat rules are tighter, which in itself might limit roleplay. But that has nothing to do with the minis themselves. And I don't see how knowing where all the creatures are standing affects roleplay. Personally I've seen some pretty neat roleplay in combat regardless. It really just depends on the player, and whether the DM is willing to allow off-the-wall combat moves.
Anyway, that's a lot of buildup for what is basically a blog to show off pictures of my minis. It's nothing special - I don't like painting minis, so I pretty much collect WOTC's prepainted stuff. But regardless, here's some pics of my collection.
Here's some of the ones my friends and I use for our characters:
Krusk (Half-Orc Rogue, LFR, Rick)
Rhogar Bloodfang (Dragonborn Fighter, LFR, Bryan)
Kryla Bloodfang (Dragonborn Sorcerer, LFR, Matt)
Voranna Elun (Eladrin Ranger, LFR, Matt
Dorath (Goliath Fighter, Praktas, Bryan)
Talindra Seryth (Shifter Avenger, Avatars of Torm, Matt)
Aria Thatcher (Half-Elf Bard, Scarred Lands, Matt)
Flint (Shifter Ranger, LFR, Bryan)
Damakos Redhorn (Teifling Rogue, Praktas, Rick)
Lia (Eladrin Wizard, LFR, Misty)
Drew Baker (Human Fighter, Praktas, Bryan)
Here's some other misc pictures of my collection. For most of these you can click on the picture for a larger version.
Other various creatures:
You shall not pass!
Lizardfolk, Kobolds and Goblins:
A few pics of heroic types and other non-monsters:
Some undead, insects, and drow:
If this one looks blurry, it's because they're using magic to distort your vision:
Okay, that was fun and all, but bring on the Dragons!
Most of these pictures were arranged and taken by KJ. Some of the props are pieces by Dwarven Forge.