Pretty simple. It's a long term store of value, like gold. As monetary units are debased, it will go up given the 21 million coin cap.
How useful is gold as a currency? Is it used in any payment systems?
Also, going back to Microstrategy. What they are doing and attempting to ramp up is really fascinating. A very compelling long term opportunity for people with money trapped in 401k or other accounts. Michael Saylor explains in great detail in the most recent earnings call starting at 28:00.
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Tl;DR
The financial products that can be provided to market based on their bitcoin holdings are actually accretive to shareholders. If you look at it as earnings, the amount of value they're creating is substantial.
Let's say they hit the middle of their "BTC yield" target for 2025 at 8%. Based on holding 252,000 BTC currently that would be about 20,000 BTC accretive to shareholders. At today's prices that's $1.4B annually.
But they will also be adding BTC intermittently via selling equity that adds to their base stack. These numbers will get big.
While selling equity in almost any other case is dilutive and knocks down the share price, it's telling that they announced the largest ever ATM equity offering and the stock is up. As he mentions, when they raise capital they deploy it and can get returns in 5 days, not 5 years.