When should one use android:clickable?

When should we use android:clickable in XML? Should we ever?
Is there any difference between such XML declaration and in-code declaration myButton.setOnClickListener? I have read the documentation, but I could not find out when and why should I ever use this attribute.

PS. I was implementing an ad SDK and found that their developers were using android:clickable with WebView and I was intrigued why did they use it.