Drawings is in many ways the Rodney Dangerfield of Google Apps. Its easy to see it as just another drawing program in which other online tools offer more functionality.
One big advantage Google Drawing has over other drawing programs or even Google Docs and Slides is its ability for interactivity. In the Alien activity the actual drawing is the spaceship in which a student can grab a number and then make a corresponding set with the aliens.
A teacher could use drawings not only for math but any sorting / counting / graphing /classification task they would like students to accomplish. Its easy to see how Google Drawings could become a staple in a GAfE environment with younger students.