The context was about having multiple skus with more internal memory, as an upsell trick, since they pay much less for these drives than the retail difference. You can notice which company is doing this, and which one have a single sku at launch. Many don't want an external drive. Some travel. Some want to reuse their SSD when they upgrade. Some want a much faster boot/app disk. Some care about replacing a broken drive without having to send it for repair. Etc...
Nothing is perfect. The ideal would have been two hdd slots and also external support. Neither are offering both. The external support doesn't remove the need for multiple skus, while the replaceable internal drive does.