What is the meaning of "they're nice to have"

I’d like to know the meaning of the phrase « nice to have » in the following excerpt from the chapter Two of The Great Gatsby by F. Scott Fitzgerald :

But immediately she turned sharply from the window and, leaning forward, tapped on the front glass.

“I want to get one of those dogs,” she said earnestly. “I want to get one for the apartment. They’re nice to have—a dog.”

We backed up to a grey old man who bore an absurd resemblance to John D. Rockefeller. In a basket swung from his neck cowered a dozen very recent puppies of an indeterminate breed