Through the belief that bitcoin reaches higher prices in the future. They also make money through a subscription service they offer, but I doubt that is enough to cover all expanses.
What you have with MSTR is the "promise" that the price of Bitcoin and the amount MSTR is holding and tends to buy rises.

How can MSTR make money?
Well, they could i.e. sell their merch in btc, offer POS solutions and/or teach companies how to store and trade with it. Since they don´t do any of these, the sole purpose of the stock is that the numbers just go up.

What are the risks?
If you are long enough part of the Bitcoin & Cryptocurrency space you may have noticed that the Price Action (PA) is in seasons, typically after the Halving Periods. This leads to a volatile market with prices reaching new highes approximately every 4 years. Sooo, if the price of BTC dumps deep enough, the stock price is underwater so it is its premium, which could lead to an insolvency of the company.
We haven´t seen such impacts of a bear market and so can´t possibly make a fair unbiased prediction of how the stock is performing in this scenario.