Why do you need expandable storage if you get 64/128GB. I've never even managed storage on my 64GB Nexus 6 since I got it in February, and I have tons of audiob

Why do you need expandable storage if you get 64/128GB. I've never even managed storage on my 64GB Nexus 6 since I got it in February, and I have tons of audiobooks with hundreds hours of length already.