Why keep a dog and bark yourself?

British English is filled with delightful bon mots. 'There's nowt as queer as folk' never ceases to cheer me up, as does hearing that someone is standing about 'like a tit in a trance,' an expression I badly misinterpreted at first hearing. But 'why keep a dog and bark yourself?' has always been a favourite, the idea being (for all you non-UK readers) that the duplication of jobs is wholly unnecessary. Or so thought I.

We have a charming pair of houseguests staying this week, two attractive young persons from Hamburg who have their own key and come and go as they please.  They reported that upon arrival, the dogs barked furiously -- so furiously that they felt quite worried. This filled me with a certain pleasure. No foreign burglars were going to infiltrate my house and get away with it, oh no. Darling clever watch dogs.

Except, continued the pair, when the dogs realized that it wasn't a member of the family at the door, they promptly lost interest and padded silently back upstairs to sleep.

Which solves the mystery of why you might need to keep a dog (two, in my case) and still be required to bark yourself.