Building on top of the features you expect from Signer, it brings additional security (and convenience) features associated with purpose-built hardware:
After initial setup your keys are never leave a cryptographic strongbox (which supports Substrate-compatible elliptic curves on-chip).
NFC connection is the only power source it needs: no wired connection needed, no internal batteries to charge.
Unidirectional, inspectable communication: NFC goes in, QR code goes out (on a high-contrast e-Ink touchscreen).
A dedicated hardware circuit detects someone messing with the chipset and wipes your keys upon the next power-on.
Translucent, inspectable plastic monobody — so you can be sure there is no extra hardware added to your device by some evil maid.
And of course it supports most Substrate-compatible chains (and their metadata updates) out of the box.
Tamper detection circuits