Many devs use place holders even for pixel art,the artist might be busy with another project but know the dimensions of the sprites so while waiting a dev will use place holders to continue dev.Being able to write the game play(handling,logic,collision) while waiting on your artist.
I studied Mortal Kombat Triology source code for almost a year, and guess who shows up when I'm planning on porting it lol,Hello DeahtAdderSF,I know who your are,My name is Richard....I'll contact you in pm