1. Having an address on your business cards makes your business look more credible and established. Think of all of those business cards you've gotten over the years with no address – don't you immediately know that those people are working at home? And don't you fear that they're working in their pajamas? Not the most professional and credible thing that can be done for a brand. You can secretly work at home, in your pajamas, with a mail box address.
Well, not necessarily a P.O. Box, but a mailbox for your business. I actually recommend contracting with The UPS Store if you have a local branch that's convenient for picking up mail a couple of times a week. They accept packages, and are usually very good about not making you wait in line to get them.
2. Have a shipping and receiving department on call to receive packages. Your UPS store will accept your packages, and let you sign for them at your convenience. If your mail man is constantly leaving you "pick up at the post office" slips at your home, or if your UPS deliveries are constantly getting you little post-its on your door instead, then this could be a better way.
3. You'll have an address to put on your outgoing emails. CAN-SPAM laws require that you place some sort of mailing address on your outgoing promotional emails – for your newsletter, email marketing, autoresponders, etc. So, instead of putting your home address on those emails, you'll have a "business" address to post.