If your doing multiple things at once can be useful. For example having someone on the phone and being able to call across the office "did we get that payment this morning??".
They can deal with the mail and be first into the office getting you open early. Because personally I prefer to work later in the day but some clients like to call at 8am.
They can process your mail, get the invoices in the post, run errands that need to be done in person. Adds a personal touch for people that visit your office.
I think it's better to hire an admin assistant locally if you're into local business marketing. It's better to have an assistant who has intimate knowledge of your target location and demographic.
I can see the following situation:
There could be more....
For me, it would be if: