OP, the best way I can think to say it, is that it allows leverage, and prevents lost opportunity.
As others have said, people may be hodlers, and wish to make a purchase, invest, or gamble, without having to risk losing their tokens that they believe will appreciate in value.
Also, for the most part, the collateral is significantly different from the borrowed funds. Think about real estate. You have a house worth 200,000 USD. You want to borrow 20,000 USD.
You could sell the house, but then you are gambling that the housing market won't spike, etc, etc... So you just get a loan, and provide the house as collateral. Nice and simple. No one thinks a thing of it.
Similarly here. If you have 10,000 in BTC, there isn't a reason to borrow 5,000 in BTC, using the larger amount as collateral. But what if you are holding doge, because you believe it is undervalued? Or --Insert useless token name here--.
Basically, you can keep your existing tokens, and ALSO achieve some degree of liquidity at the same time.