[richardknop]

It depends. But one of the reasons I can think of is if it is an early stage startup or small company. They might be using co-working space like WeWork or only have a virtual office and couple of employees working remotely/from home.

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.