There are many things that are useful to a writer in their craft. One that is not as obvious as many is a dog. You might wonder what possible benefit a dog can bring - in fact there are several.
The first is company. If you are a full-time writer, the chances are you will spend a lot of time alone. Dogs give just the right amount of presence without getting in the way of the writing process.
Second is exercise. Writing is, inevitably, something of a sedentary occupation. Having a dog about the place forces you to get out of your chair a couple of times a day, come rain or shine, for a walk. This might not seem so welcome when it's pouring down (the last few days we've had sun and it has been glorious), but the dog's great strengh here is that it forces you to get past the 'can't be bothered when it's so miserable' syndrome. You have to take that walk.
Finally, there's creativity. The dog itself doesn't do a lot for this, but it's a side effect of those regular walks. I'd say at least half of my best ideas for books and things to put in them have been dreamed up while out walking the dog. It's essential for this to work that you don't take a mobile phone, that just causes distractions. With either a notebook or a recording MP3 player to jot down any thoughts you've got yourself an idea factory.
Of course, a dog is for life, not just for writing - but they genuinely help.