There are plenty of software programs and apps that can use facial recognition technology to perfectly crop out the subject of an image, but the Pasted app intentionally messes up the precision of such algorithms.
Though there's something to be said for those perfectly cropped faces, such accuracy can often come off as robotic and excessively clean when used in collages. The Pasted app instead opts for a more classic aesthetic, using a system that mimics the paper-and-scissors cuts that accompanied physical collages from that distant, almost-forgotten time before digital media.
The Pasted app still uses facial recognition, but it essentially provides a rougher cut that includes some of the background, similar to the cutting that one would do with scissors. Users then have the option to further edit the crop as they see fit.