The dual power adapter is in addition to, rather than instead of, MagSafe. The cable has the magnetic connector on one end and usb-c on the other. Dual ports just lets you charge your iPhone/iPad etc. at the same time.
I can't see the value in getting the 2-port adapter over the 67w fast charge one. You could always charge your phone through your laptop if you really don't have a second power brick.
Size. It's not apparent in pictures but the 67w is the size of like a deck of playing cards. The dual port is a tiny Gan charger.