What do these magical things do?

Ultrasonic cleaners remove corrosion that you can't see. Many chips on the motherboard are BGA - ball grid array, meaning a plate sitting on top of a bunch of marbles. You can brush the plate and around it, but how do you get rid of the dirt on the center marbles under the plate? Even if you manage to clean the outer sections with a toothbrush, you will never get the corrosion underneath the very center of a large chip. An ultrasonic cleaner uses a process called cavitation to create thousands of bubbles that get under everything, exploding on impact and removing corrosion, dirt, and other junk that will keep the board from working. Simply put, if you're in the business of electronics repair where liquid spill is a problem you see: you need this machine.

For iPhone Repair:

We recommend the Crest P230H-45. Cost effective, and will fit any phone board you put in it. All the cleaning power of its larger counterparts in a smaller package. 

For Macbook Repair:

We recommended the Crest P1200H-45 because it will fit all Macbook boards, even the newer Touchbars. We used to recommend the Crest P500H-45: however, with Apple's release of the Touchbar models, these new boards do not fit properly into anything smaller than the Crest P1200H-45. You can try using the P500H-45 without a basket and cleaning certain parts manually if on a budget, but using the P500H-45 without the basket to fit a touchbar board will eventually damage the elements of the cleaner.