Good question.
Perception. Apple wants to change the perception of what the

watch is. Because, as you may have heard, smartwatches are very hard to sell, no one seems to really need them. They are a solution in search for a problem.
Most people are simply not that eager to have yet another tech device on them. So Apple is a tech company doing something unusual : convincing their market that this not just yet another tech device, but a beautiful piece of jewelry enhanced with technology, and not the other way around. Meaning : "if you're not in the market for a smartwatch device but just looking for a nice watch, you might want to look at ours. And we'll throw in some sexy tech to sweeten the deal."
Therefore you need the Chanel and Lagerfelds of this world to help pass the message.