Speaking as someone who has purchased numerous slabbed books... The slab is just a container like Jim said. I love cracking a slab open and pulling out a book from the 40s with white paper, as the label says, or holding in my hands a book that went from the original owner to the dealer who purchased the collection to CGC. Their grading, while not as consistent as I would like, is within a grade of the actual condition, 99 out of a hundred times. Grading is relative, and subjective, meaning that a defect that I personally can't see in any grade higher than a VG, someone else may consider allowable in Fine, and any grade is an assessment based on other books, and comparable defects, within the graders experience. CGC has screwed the pooch on a fair few books, but they've also graded well over a million of them, and have remained consistent enough to maintain a loyal customer base, and a decent reputation in the high end market.
Boiled down, CGC is good if you are buying a book for investment, or looking for a book of a minimum damage quotient. Also, in the event that they do miss a centerfold being gone, they are the Certified Guarantee Corporation, and they have historically been very good about backing up that guarantee when they have made an egregious mistake.
But, where they really shine, and the reason that the high end market loves them so, is in their restoration detection and disclosure. A PLOD (purple label of death) can and should mean the difference of thousands of dollars, and having a CGC Universal blue label on a high end book makes for extreme liquidity to those who love the books for their market value as much or more than their aesthetic and intrinsic value.
Basically, CGC is necessary in an online market environment, where you can't hold the book in hand and make paper quality and overall grade assessment yourself.
If all you're looking for is a reading copy in passable shape, no, they are probably not essential. But, if you like having nice books, with no hidden issues, and like to know about any restoration upfront, CGC is a fairly reliable barometer of whether or not the book will make you happy.