TL;DR: It speeds up the post saving process and ensures only changes fields are updated in the database. Long version: The plugin detects changes made within selected post types' forms and passes this information to 'acf/save_post' hooks, where data received from the form is processed. All values that come from unchanged ACF fields (not detected as changed in the UI) are ignored. As a result, only changed ACF fields are saved in the database. This improves the performance, especially in the case of very big forms (with lots of ACF fields).