Keeping the peace when independent operations can make their own giant robots. Staying somewhat prepared for returning existential threats.
Stowing expensive purpose-built machinery. Aircraft graveyards. The economics of storing and securing out-of-production technology for later use.
Securing mobile war machines. EMPs. Resistance to signal-jamming. Broad attack surfaces.
IRL progress towards a brain in a jar: human brain organoids implanted on a mouse brain. Sensory apparatus and feeling pain. Getting (ethically) hairy.
Actually: not so rare! But a pain in the ass to extract. The Japanese rare earth ocean-bottom treasure trove and how it will be difficult to access. Mining rare earth elements from spaaaaaaaace and how we probably need a new colloquial name for these materials. Underwater volcanoes.
Magical Jaeger blood rocker fuel. Gravity gun space race. Keeping Earth as a farm for kaiju blood, fueling our expansion into the entire solar system. Throwing rocks at things for fun and profit.