temporarily increase the price so tech-bros (re: Elon) can sell at a massive profit.
Actually it is similar to gold reserves (bitcoin is better) without advantaging competitor nations that have gone in on gold reserves already. There is an energy/mining motivation aspect as well as a populist widespread participation.
In terms of "real purpose", US debt can grow much more unsustainably, and there is something to fall back on when a default occurs. The tech bros that helped Trump do want to use other crypto projects for fintech and other innovation, rather than pump it and dump it.