In the teaser trailer for Spider-Man: No Way Home, Doctor Strange is wearing the Eye of Agamotto, a mystic device that acted as a container for the Time Stone. However, the last time the hero was seen with the locket was back in Avengers: Infinity War, when it was destroyed by Thanos. The Infinity Stones were destroyed soon after, which gives Strange little reason to carry the Eye anymore -- well, unless it serves a greater purpose or there is something more mysterious at play in the next Spider-Man film.
A Decoy Made To Protect The Universe
The Eye of Agamotto that Strange is seen wearing could be a replica, one made not for sentimental purposes but for a tactical reason. It makes strategic sense for Strange to keep a decoy of the locket on him, as few people know that Thanos destroyed the Infinity Stones. And by keeping up the illusion that he still has the Time Stone, Strange is able to maintain a powerful presence against the MCU's enemies. If it became widely known that the Time Stone is gone, major threats like Dormammu will have their way with Earth, with Strange being severely limited in ways to stop them. In short, Strange may need the Eye back in order to keep up appearances that Earth (and his dimension) is still protected by the same gem that the Sorcerer Supreme has kept safe for generations.
More Than One Eye
It could also be the case where the Eye of Agamotto is much more than just a container for the Time Stone. In the comics, the Eye carried by Strange is one of three created by Agamotto -- one for Truth, one of Power, and one of Prescience. And, in fact, the Eye of Truth can be used in the astral form and has several abilities, including an all-emitting light, playing back recent events and mystical telepathy. So, outside of carrying the Time Stone, the Eye is a powerful device that Strange can make useful in many circumstances, meaning it would make sense for him to continue to wear it.
As the Sorcerer Supreme, Strange has a duty to protect the universe from all sorts of mystic threats, a task that is made much harder without the Eye of Agamotto and the Time Stone. By keeping a replica or wielding a second Eye, however, his enemies will think that he is still in possession of two of the most powerful items in the MCU. And this is particularly important while he is traversing the multiverse, as, without the Eye, others may know that his universe is vulnerable.
To see why Doctor Strange still wears the Eye of Agamotto, Spider-Man: No Way Home swings into theaters on Dec. 17.