I think it's similar to small construction companies using their staff during the winter to work on the owner's new rental properties.

They can easily scale it up or down depending on how other projects/priorities are going and how many people don't have tasks to do. All of the deadlines and KPIs are internal and they don't expose themselves to lawsuits or financial penalties when they have to delay something.

In the case of spacex, it's even more beneficial because they need to launch rockets anyway for testing. Sending them to space empty is a wasted opportunity.

Starlink has massive DoD potential, like it or not. Also could be big for banking. Their client facing aspect of it will eventually just be piggybacking off of those two industries.