Because, with any decent employer, that laptop is probably more powerful than anything they could afford.
I know I use my work laptop at home almost as much as I do in work. And when there is an emergency (about the only thing I would work from home for free for), I can just log in remotely and have ALL the tools I have in work just sitting there. Because of quite open and flexible policies, it means I can check my work email at home and my home email at work, too.
Also, it's a laptop. If you have one, it's because a desktop wouldn't be suitable (I know this isn't true for a lot of people, but it's true for myself) because you *do* need to move it around. This is why I have a laptop and not a desktop despite having an office to myself - because sometimes I need to lug it around and do things on it elsewhere (even once inside a crawl-space) and having remote-access to a desktop wouldn't be anywhere near the same in those circumstances.
I certainly don't think of my laptop as a status symbol. Hell, I spend most of the time it's off-site trying to hide the damn thing even though I never leave it unattended.
But since I've worked somewhere which has a policy of letting me buy things to my specification, I haven't bought myself a personal computer. It wasn't planned, only incidental, and I've asked that, should this laptop be replaced (we have a 2-year PC replacement policy) I be allowed to keep it rather than it filter down to our "second-hand use" pile. If I hadn't broken it's predecessor's screen-hinges through over-use, then I'd have kept that too. Both were the most powerful machines I'd ever used at the time.
And my employer benefits - I get decent tools that I'm much more familiar with, I do take care of the machine a lot more than I would a work desktop, they do get their money's worth of use out of it, they do get a remote IT manager with all his tools available at all times for emergencies (though they know that if they abuse that, it stops), they do get the occasional benefit such as me pulling in code from my personal projects into work projects and/or working on work projects when I think of something useful to add (and, yes, I have clarified the intellectual property situation - work done on my projects is mine, work done on their projects is theirs), and it doesn't cost them any more than a "not-off-site" policy (we already have accidental damage warranties that cost next-to-nothing and barely use them).
A lot of our staff take their laptops home. It can even be a bit of a pain to get them all back in for upgrades / replacements so we have to give notice when that happens en-masse. But it means that people come to rely on them, thus take more care of them, and they have no excuse for not having finished their work ("I couldn't get in" or whatever), and the extra associated cost is minimal. It also means that the workplace has much less value to a robber! (We've yet to have any thefts of staff laptops, but we have had only two breakages over three years, so it's hardly a chore). Everything is security marked, encrypted, and traced anyway, not to mention worked into the policies as the property of our employer, but we expected a lot worse when we started.
We get the odd rogue program installed (e.g. some game they've bought normally) but, to be honest, the staff don't have admin access and don't need it and they hardly even notice. About the only thing I do for them is the occasional Flash upgrade so they can watch BBC iPlayer. Everything else, they are happy to install / run as their normal user even at home.
And with TrueCrypt installation, and the fact that I'm the only one who can actually take any data off-site (and not just my own work which staff would be emailling / taking home via USB key anyway), and being the responsible IT guy (so it would be my head anyway), they know their data is pretty safe. But, yes, I take it home a lot, I've watched movies on it, I've taken it on holiday on the plane, etc. You might think I'm being showy, but it makes sense for everyone involved.
Value your staff, trust them and provide them with decent tools and they will take care of them. Treat them like children who can't be trusted to have anything and they'll act like that out of spite. And the laptops we buy for staff (mine's a bit of an exception) are pretty much the bog-standard cheap business laptops - but you don't see them complaining because it means that *they* don't have to fork out for them. A lot of staff enjoy the freedom of taking their laptop home, even when they have iPads and iPhones galore at home of their own.