The only interest that ever makes sense to say it's a good thing, is the amount you'll collect if you're a long term holder of Bitcoin. Loans never really make sense if you can avoid them. It's basically signing up to lose money, so that you can have something sooner. If I offered you a pizza today, but you'd have to overpay 2x the amount of what it's worth, instead of waiting to have it in a week, you'd probably not go for it.
I too do not how tje idea of loan, but I remember I first saw what is called 'buy borrow die strategy' on BBC few months ago, I couldn't get the link right away but here is another
https://clacified.com/business/16/financial-hack-the-buy-borrow-and-die-strategyThe three is said to be commonly used by very rich Americans. The borrow aspect of it, if compared to bitcoin, a rich man can presume when there is some corrections in the price of bitcoin, he can be able to use some of his bitcoin holding as collateral and expecting the increase in bitcoin price over 2 or 3 years.
According to what I have learnt, the tax paid for spending bitcoin holding by the rich people is much more higher is compared to borrowing, and additionally, fiat do depreciates in value because the government makes it centrally controlled and inflationary.
The rich people will have experts to let them know if it is a good idea to use asset as collateral (bitcoin is an asset) and if it is the right time to use it. But just not making a correct long term analyses for it and just go and borrow will be a stupid idea.