There are two main reasons why a small business might want to get a US mail drop address:

1) you work from home but you don't want to put your home address on your website or business cards
2) you live outside the US and you want to order something that is only available to US addresses

It's fairly easy to get a US mailing address - there are various services that will act as a mail drop and even include phone/fax facilities like a virtual office. Another advantage is that if you want something that's only available in the US you can get it sent to your "US address" then redirected to your home address - very useful.

One of the bigger ones is USAMail1.com but if you do a search for "USA mail drop" you'll find several to choose from.