In such case they want to appear bigger than they are and if you could google their address and find out it's a virtual office with 200 companies registered there you might decide to look for a bigger / more established company instead of going with them.
But even if your company's address is just a virtual office, you should list it visibly on your website in my opinion. It doesn't look good when you appear to be hiding it. There's nothing wrong with small business or new startup not having a proper office from beginning.
Logically, it makes sense as websites usually get updated only once in a while and it would foolish to dedicate a resource to it. But, it also adds a great difficulty in building a tiny thing, even if something like a contact page.
I guess big companies have more organised processes and teams to handle this. But, in startups, everyone is already working on the core product and everything else seems like a distraction.
But I agree that not telling prospective employees where they're located will deter people from applying. For example, two places within a few miles of each other in NYC can have drastically different commutes (time, comfort and/or safety), so there's no way I'd consider applying for a job if I didn't know the company's location.
Since we were basically working out of a house for the longest time, whoever was on hand would have to deal with the unwanted guest. If you're alone in the office and some big dude comes in saying "Well I won't leave until I meet your founder!", that can be scary.
As Gustomaximus puts it, it is to keep potential lunatics away.
I can understand small companies, trying to cover up the fact that they are still operating out of their garage or spare room. But when large corporations do it, I am just flummoxed. For example if you look up Apple in the Sydney (AU) whitepages, there is no address given just a free call number. I just checked, Google now show their address, but in the past they didn't either.
For would-be competitors who want to send in spies, it's easier to move on to other companies that publish addresses.
In the big 5 companies, the more hush-hush the R&D activities, the harder the particular site is to find.