techchick - back to the ‘what is this stuff good for’ part of your post. Race, gender, marital status, veteran status, income, hours worked, vehicles, bedrooms, all that stuff.
Easy, cop-out answer: ask Congress. They’re the ones that passed laws requiring that this info be gathered by the Census Bureau.
For that reason, I can’t tell you the real reason why the questions are on there. I can give you what I find to be a fairly good justification, though. I expect you may disagree with most of it, but it would be kinda boring if everybody always agreed, right? 
It’s worth remembering, before we jump in, that this data won’t be available for individuals or households, but only in aggregates.
Yes, the government has a lot of this stuff already. What they don’t have is the info in a way that is (a) nearly as complete as the Census can get it, or (b) in a form where one data variable can be correlated with others. And, of course, (c) some of this stuff (e.g. hours worked) may not be collected anywhere else, for all I know.
(a) Completeness: the Census can get information from parts of the population that don’t show up much on commercial databases, due to poverty, ob, or whatever. We may have various records that suggest the approximate ratio of men to women in various age groups. But after collecting data from 98% of the population, you pretty much know for sure, and you can find out if data from more limited sources has biased our samples.
Is this information useful? Sure, if you’re going to plan an elementary school or old-age home, or a whole bunch of other things.
(c) Stuff that may not be available anywhere else: how about those hours worked? Is this irrelevant to matters of public policy?
To me, they’re very relevant. Child-rearing has never been simple, but as it becomes increasingly complicated in a complex world, we might want to know if people have time left over at the end of the day to do the job right - let alone keep an eye on their elected representatives at various levels of government, which is part of the responsibility of every citizen.
I’ve got a friend who’s left Lycos and is negotiating with three different high-tech companies for a new position; he’s trying to bargain for a 60-hour week. Is he in an unusual position in not being able to combine that sort of career with a normal work week, or is he part of a larger trend? After the Census, I expect we’ll know. But it’s silly to say, “I think people are working longer hours than they used to, because I have this friend…” I want some numbers to back up my hypothesis, or kill it, whichever.
And (b) the stuff can be correlated. What’s the relationship between income and hours worked? What sort of demographic groups buy what sort of vehicles? And since everybody gets access to this information, it gives all of us a leg up in what we’re doing, whether it’s you doing market research for starting a business, or me being an armchair social scientist.
I have a hard time seeing what can provide more sheer opportunity for people, here in the year 2000, than the easy availability of useful information. Somebody’s got to gather it, and it would be appreciated by most of us if that ‘somebody’ had a track record of protecting the privacy of the individuals whose information made up the aggregate data being made available.
In the private sector, we haven’t seen the last Doubleclick. But the Census Bureau has a long history of fighting (successfully, I might add) to protect the privacy of individuals’ data from the IRS, the INS, the FBI, you name it.