These resonators save a little hassle and space in building your own breadboard or protoboard Arduino clones. They’re not as accurate as crystals but this only translates into a few seconds to a few tens of seconds a day of inaccuracy. These are the resonators that we use in the BBB & RBBB.
Most microcontroller projects really only need to be concerned with relative time and not absolute time, and resonators are fine for relative time. Absolute time is a little problematic on the Arduino even with a crystal and best done with a real time clock chip (RTC).