For example, a bank only agrees to hold onto your money, because it will actually spend it for you.
Whenever you "saved" money, unless it was in your own coffin of gold, you actually just gave it to someone else to spend it with the promise that whenever you want it back they'll have it.
That said, if the market crashes, they won't have it. Because the bank's plan to "have" your money when you ask for it is actually to give you someone's else's. Cause your money has already been spent, either loaned to someone who wanted to buy a house, or start a business, etc.
That means if all those loans defaults, the bank actually won't "have" your money, it'll have lost it.
This is at a high level (from my understanding), why the government always bails out banks and bank related markets like home loans. If the bank go broke, everyone's money is gone.