The I-94 says on the back that, when you leave the US by air, you hand it to an airline employee (typically the check-in reps or gate reps) when you leave the country. The airline will have a process that will get the I-94 back to the local USCIS section, who will enter it into their computer system, updating your record to show that you are now no longer in the country on a non-immigrant visa.
K-1 entrants who have an AOS interview typically give (or have taken from them) their I-94's at their AOS interview.
When a K-1 entrant who did not have an AOS interview (and therefore still has an I-94) goes to temporarily leave the US after receiving their green card, it is probably a good idea to follow the I-94 instructions and surrender the I-94 to the airline rep upon departure. If this does not happen, USCIS will continue to retain a computer record showing that you are still in the country. This may cause the CBP officer you encounter at the airport upon your return to become very confused, as his computer will be telling him you never left the US after your K-1 entrance, but you will be telling him you have left the US and wish to reenter with your green card.
I've said it before and I'll say it again: it is usually very unadvisable to take any course of action that is likely to confuse a government official . Government officials, when confused, can start doing random, inconvenient things that generally make your life more interesting than you'd generally like .
