In CS this could be having gained entry to MIT or having worked at Google. In these cases you have an institute that you respect saying "we've evaluated this person previously and they've met our high standards". So regardless of the fact that went to MIT or worked at Google, the very fact they got in conveys valuable information to the future employer.
I'm willing to bet that if you applied to hundred companies with two applications both otherwise identical but one having a MIT degree and one having a state college degree, the MIT degree application would get you far more interviews.
Also there's the fact that schools are different. There are plenty of college which are essentially "java schools" which have very minimal coverage of CS beyond programming, which is significantly different from what you'd get at somewhere like MIT. The same applies to MBA schools, but even more, there's no standardised curriculum and the quality of the teachers varies far more, you could be taught by the person who invented in industry the business practices he teaches, or alternatively by a teacher with no real interest in the subject and no industry experience.
There's a lot that can be learnt from books, but there's a huge amount of knowledge that can only be learnt from people with industry experience and that's a rare thing.
If you go to one of the top MBA programs then you'll meet some of the people that will be hired by other big corporations and you will be either competing against them or negotiating with them.
However, assuming you don't have (or can't show me that you have) the skill or experience I'm looking for yet (this applies to everyone who is just out of college, who didn't do significant work as an intern or significant project work, e.g. most recent grads.) I'll take a programmer from MIT over a programmer from woodland community college.
However, this is different from just getting your first job. Lots of people under perform. Sure, if I'm hiring sysadmins, I have a pretty good idea who's good and who's not. But if I'm hiring outside my area? I can't tell a good physicist from a bad one. hell, I can evaluate basic C proficiency, and I can search for kernel patches, but if I was hiring a kernel dev, while I could probably filter off the completely unqualified, I wouldn't know the really good from the mediocre. In those cases your school still counts, especially during the early parts of your career. (in the later parts of your career, the prestige of previous companies you have worked for takes some of that.)
Generally, hiring outside of your area is a bad idea and best avoided, but it happens. Some of that is necessary. I mean, I /need/ an accountant for all the same reasons why I'd be bad at evaluating an accountant.
Now, even then, school isn't the be-all and end-all... I chose my accountant because she ran a free question-and-answer blog[1], and seemed to know what she was talking about, and as far as I can tell, I chose well.
Still, I think that if you are going to school for the 'accreditation' value of a diploma, a diploma from a top tier school is worth rather a lot more than a diploma from a place I have not heard of.
Then, it's because business expertise and talent is hard to verify. Programming or engineering ability is easy to prove, even at entry level. You can't prove you can manage a large group until you're already in charge of a large group. This means a company is taking a big risk on hiring MBAs versus CS grads, and needs to look for some clear, objective signals before going ahead.
Finally, executives like hiring managers who went to the same school they did. I don't know exactly why this is the case -- reciprocity, maybe, or some notion of a shared vision of how to run a business.