Read-only feature:

Receive a useful items depending on the amiibo / After beating the game: Unlock a bonus or feature depending on the amiibo

Scan these amiibo to receive these useful items:
- Samus Aran (Samus Returns): Get Aeon Reserve Tank (refills gauge when empty)
- Zero Suit Samus: Get Energy Reserve Tank (replenishes health when empty)
- Any other Samus amiibo (Smash Bros. and others): Get Missile Reserve Tank (replenishes missiles when empty)
- Metroid: Place a Metroid Marker (see closest Metroid on map)

Additionally, after beating the game, scan these amiibo to unlock these bonuses:
- Samus Aran (Samus Returns): Unlock Metroid II art
- Zero Suit Samus: Unlock Sound Test mode
- Any other Samus amiibo (Smash Bros. and others): Unlock concept art
- Metroid: Unlock Fusion difficulty mode and play with Fusion suit

Sources: Official Japanese amiibo website, Reddit / Thanks: Matteo