So you basically don't invest in anything because that theory can be applied to any asset.
In general, anything that turns a raw material of some sort into a more refined product (be it iron ore into steel, steel into machines, or electrons into advertisements) is adding economic value into the system. By doing that work, the total net wealth of humanity increases. The economy is positive-sum, and so investing in anything that's correlated with the economy as a whole should be worthwhile.
Bitcoin doesn't work that way. Trading Bitcoin isn't adding to the total net worth of humanity. Mining Bitcoin arguably could be, because the ability to make transactions has some value, but the practical value of those transactions is less than the value of the electricity, computer hardware, and pollution that go into making them. Speculating on Bitcoin does nothing productive; it's zero sum. Net-zero systems are nearly indistinguishable from gambling unless you have insider information.
One that comes to mind especially for Bitcoin is that it “could” act as a global store of value, independent of any one geopolitical power. Many countries lack stable and non-corrupt currencies, and many people in those countries cannot trust putting large sums of money in their economy, for fear that they will be taken away from them. If BTC stabilizes in value sufficiently, people in those countries could use it as a store of value—like gold, but easier to obtain. Id argue that the idea of having a digital store of value does increase the productivity of the world, because people don’t have to rely on inefficiently hoarding gold, or paying fees for buying gold in the stock market.
Of course, it's very unlikely that anybody in this thread bought them because in reality we are looking for gains correlated to hype/market performance/inflation/interest.