Eh, not so much - if the water table isn’t too high it will keep it out for awhile, but not forever. New Orleans has mostly above-ground vaults because their water table occasionally is higher than ground level (hence all the pumps) and keeping the bodies above ground keeps them (relatively) drier longer.
About the only really practical thing about the vaults, as I understand it, is that they tend to keep the ground from collapsing as the bodies/caskets decay and collapse. How fast that would occur and the details varies depending on ground conditions, among other things, but old-time graveyards in Europe were known to be hazardous places to walk what with the ground either humped up over fresh graves or sunken over old ones, and at times the footing could give way under you due to all this exciting post-mortem action. As a side effect, a burial vault can delay the deterioration of the casket.
Whether or not they are actually needed depends very much on where the burial is. Also, whether or not they’re required depends on local regulations as much if not more than actual need dictated by soil and climate.
It’s not unusual to be told they are “legally required” even when they aren’t, just as you might be told embalming is “legally required” even when it isn’t. For example, in my area if burial or cremation occurs within a certain time limit of death no embalming is required. Now, if the funeral with body is to take place beyond that time limit then yes, it might be necessary but people should be told the actual rules and options so they can make an informed decision.
AAA digression:
Hmm… largely, I’ve managed the dead batteries and flat tires on my own (I’m handy like that). I do know a local tow service with reasonable rates I’ve used for other problems, including one time at 1:30 am when they arrived reasonably fast. So locally no, I don’t need AAA.
Now, when I take a long road trip with a rental car I do get their insurance and road assistance for just-in-case (my own car insurance does not cover me for rental vehicles - I do get minimal insurance for myself).
So it’s not so much “charmed life” as having an alternative to AAA for those purposes. AAA does provide a service that some people want and need, but not everyone does.