Kara's necklace: Plot Device or Red Herring?

It was used to show the depth of her love for him.

He'll return it to her but how it will be used depends on their plan for the couple and how Mon-El returns.

If there's a search for him, finding the necklace could be a clue of his whereabouts or to trace his steps.

If he returns with memory problems it can be used as an identifier.

If there's no drama surrounding his disappearance and return, he'll simply return it.

She gave it to Alex to hold at the end of Season 1 finale and then Alex gave it back to her. Maybe it was just a sign that Monel would be coming back.
When my mom died a few years ago I wouldn't have given the last thing I had of her to anybody. So it was a little puzzling to me.

She gave it to him to keep him safe, as her mother gave it to her.

People do that all the time....Mother's giving their daughters something their mom gave them....

Mother's giving their son's an heirloom to give to their fiance as an engagement ring.

People do this all the time, and all the time in movies, tv etc. One example that comes to mind was the Christmas Movie about Dolly Parton "Coat Of Many Colors" where Dolly an heirloom from her mother, both of her teacher's parents had passed away. People do that to show their love that their parents gave them and pass it on to someone else.

Giving Mon-el the necklace was to show him just how important he was to her and that their love would cross the distance of space...I thought it was beautiful.

Will it come into play later? There are all kinds of things they could do with it. He ends up in not the Phantom Zone, but maybe the same zone where Kara's mother and father are, and he has her necklace so they know that Kara is alive and well....Kara's father makes a cure for the lead poisoning rather than Brainiac and we move into a possible "New Krypton" story line or something like that....

I think it was both meant to imply that he will be back and to be used as a plot device. I'll be pretty disappointed if it isn't used in thier reunion in some way. At this time I'm not going to speculate on how it will be used just that it will.

That necklace is the one physical thing Kara has of Alura's and it's meant to come back to her. She gave it to Alex so if she died Alex would have it but it was right that Alex gave it back.

Maybe it will become a running theme that she gives the necklace to somebody else at the end of every season