It's not a gain if you don't realize it.
To the extent that we care about the distribution of net worth, though, unrealized gains are an important part of the story.
[0] https://voxeu.org/article/consumption-and-income-inequality-...
This only costs you the interest of the loan and exposes you to the risk of declining value in the assets securing the loan. Appreciation of the assets or dividends may fully offset the interest or more.
Additionally if you a founder, for example, you retain the influence/control of your company that you derive from the stock ownership, while still be able to enjoy their cash value.