The key atmotorway is used for amenities that are mainly intended for motorway users, like gas stations or restaurants.
Using this tag those amenities can easily be queried and rendered on motorway maps.
A guideline for whether to add it or not can be if it is signposted on the motorway or not.
It can be combined with a motorway_direction tag if it it only accessible from the motorway in one direction, which would default to 'motorway_direction=all', and supports the same directions as Key:direction. It can then be rendered with a CartoCSS marker-transform to render them in a Michelin map like style.
This key is proposed at Talk:Tag:amenity=fuel