When do you buy a book physically AND digitally?

Title. I'm currently reading the entire Oz series (only 99¢ due to it being in the public domain) on my Kindle and I found a really cool paperback complete collection, all 15 books, for around $50. It's a solid deal, especially for how pretty the books are. That said, do I double-dip? When have you bought both versions? Thank you in advance. :)