It probably boosts your chances of admission at a given school slightly, but the negatives almost always outweigh the positives. Once you're accepted, the school is no longer competing against anybody for your business, because you are committed to going, and thus the primary reason a given school has for offering gift aid (i.e. grants and scholarships) is removed, so you can expect to receive approx. $0 in non-need-based aid.
Near as I can tell, the only way it makes sense is if you're applying to a distant reach school where your chances of admission would be very low otherwise.
For example, if you had a 3.5/162, applying ED to Michigan might make sense.
(note the difference between EA and ED. If possible, always apply EA.)
The counter argument is something along the lines of 'well, I REALLY want to go to school Michigan, so as long as they admit me I'll go anyway, and this way I have a better chance of getting in'
Fair enough, but lets say your numbers are pretty average for Michigan - 3.75/168. Judging by LSN, an applicant with these numbers has a pretty good chance of admission without ED, assuming he/she submits his application fairly early.
Now, as we established, said applicant only wants to go to Michigan. The school is by no means a lock, but it's not much of a reach, either.
In this scenario, it would probably make sense to take a calculated risk and apply regular decision to Michigan and 10 other schools. Here are the pros and cons:
Pros: bargaining power. Assuming you're admitted, Michigan doesn't know you'll go there for nothing. Once aid packages come in from other schools, you can run them by Michigan, and probably get it to raise its aid offer. In other words, you could trigger a bidding war. Also, there's always that chance you get admitted to NYU or something, in which case you might decide that UMICH wasn't quite the dream school you thought it was. Had you gotten accepted ED, NYU would no longer be an option
Cons: slightly lower chance of admission to dream school
EDIT: although I've seen some posters argue that applying ED doesn't do much for your admissions chances, I don't think that's true. Since an ED admit requires no gift aid, my guess would be that the auto-admit range for those applicants moves to approximately the mean LSAT and GPA numbers for the previous year's incoming class (again, just going by the last few years of LSN for this).
Of, and in answer to your other question: there is no God, no, not one.