I recently explained a related question like this, so here it is for you as well:
1-4 Series Cards; NPC villagers can give you their photo (once per-character). Regular villagers can be moved in on demand. If you are not doing either, you can ask for a random item. I am not sure if this is 1-4 series and misc. promo exclusive, as I never would use the option for other types.
Welcome Amiibo Series Cards; Will summon an RV at your campground. Villagers can be moved in on demand at their RV.
Sanrio Series Cards; Will summon an RV at your campground. Villagers can be moved in on demand at their RV. They cannot be moved in when summoned by outside towns.
Misc. Promo Cards; Functions the same as 1-4 series cards.
After moving in RV Amiibo villagers, you can no longer summon their RV. Scanning villagers in will negate the villager cycle. The fewer Amiibo villagers you have, the less often they will want to move. You may make one Amiibo request per-day per-town.
As said above, they can be used to summon villagers in HHD to build their homes or summon them on other lots. This only works with series 1-4, misc. promos, and figures; anything else will crash the game when scanned! Some cards and figures unlock special items when scanned.
Hope that helps!