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Why would Tesla want Model 3 reservations?

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When Tesla receives a reservation for a model 3, they have a idea on how many they will need to gear up to produce.

When they are configured, Tesla knows about how many components they should contract for. Different color seats, battery sizes, wheels, accessories etc.

A supplier might be much more willing to contract with seats or other items, knowing that there is a backlog of orders just waiting to have their cars built.
 
Tesla needs money upfront to cover cost and other capital expenditures for the rest of their business. Cars won't be built until after the Gigafactory is online, which I suspect will be in 2H 2017. I doubt Tesla will commit to any orders with suppliers until 2+ months before manufacturing lines are fully up and running.