Novel marketing heresy

I realise I’m about to commit novel marketing heresy here… but bear with :)

I’ve listened to Thomas’s podcast episode on book metadata 1.5 times…

…and I still don’t understand in practical terms why I need book ISBNs.

I'm willing to get them anyway :) but I would like to understand why it would make a difference to my publishing business.

And - on that topic, can you ISBN a book after it’s been published for several months on Amazon KU?

And if so, where’s a good place to get ISBN’s?