Why do people buy expensive digital streamers?

I see that Naim and Linn streamers are often recommended on hifi fora, but why? Apart from them looking nice and perhaps fitting in with the rest of ones kit, what do they do that an Apple Airport Express can't? There'll be no difference in sound between the cheapest and most expensive streamers, bar whatever tiny difference the various DACs might make. Also, why do Naim offer extremely expensive external power supplies for their streamers? Such things have no bearing on sound