YSK that if you have an iPhone and a MacBook you can copy something on one device, and it copies it on the other device as well.

Very helpful for email addresses, say if you copy it on your phone but want to send it on your laptop because you are typing a long email. Also for notes and stuff, or if you have a calendar app that doesn't sync automatically, you can copy the text of an event and paste it to the other. Also passwords and stuff like that. Hope you knew this :)