1. Logistics: Cheaper to ship to rich Middle east and maybe even Europe
2. Risk balancing: With uncertainty at political levels about tariffs and trade rules, it makes sense to distribute production to multiple places in the world.
3. New demand: Lower cost of production == lower prices in India. Apple has negligible penetration in the largest fastest growing market. This move is a bet to try to beat android competition in price and raise global demand for IPhones.
Honestly, this move is a no-brainer. The benefits are endless.