It gives a lot more purpose to the whole note - taking aspect of ebooks when your able to easily access the notes outside of your device and book, which is surprisingly uncommon. (blog.the-ebook-reader.com)
With both of those cards reformatted to FAT32 outside of the device, the SanDisk could achieve 60 MB / s read and 45 MB / s write speeds while the Samsung could only muster 50 MB / s and 38 MB / s, respectively. (thewirecutter.com)
If your key was stored outside of your device which in some ways more secure you would need an external device (google now has enabled encrypted usb keys for ultra security minded) or possibly in a cloud (lots of proponents to that one because the user can not physically secure the key) but as mentioned if you have no access to the cloud you can't retrieve your key to use your phone. (9to5google.com)