Okay, an alien warlord threatening to lay waste to Earth is probably enough incentive for Doctor Strange to get involved in something not directly magic-related, but if the Mad Titan steals one of his sources of power, that provides extra incentive to jump into action.
Aside from his magical talents making him a useful ally to Earth’s Mightiest Heroes, there’s also a possibility Doctor Strange has something Thanos wants.
If that’s the case, then Thanos will be paying him a visit to snatch that Eye away from him.
This has led to speculation that the Eye could actually be the Time Stone, one of the two Infinity Stones not yet introduced in the MCU.