Mayhaps an example.... Pick your favorite commodity, how's tulips, I like tulips.

I deposite a bitcoin in your bank, which on the day I deposit it happens to be worth a tulip bulb. You charge me a fee. You take my fee and, with some black box inhouse currency mumbo in the middle, buy something less than a tulip bulb (you only have less than my deposit in fees, and my deposit is worth a whole bulb, so you can't possible buy a whole bulb). Over time, your less than a whole bulb appreciates and becomes worth a whole bitcoin. You sell your bulb, and now have a bitcoin.  You pay me a share of the gains in interest.  I now have a bit coin and then some.  Sadly, my bitcoin and then some can't buy a whole bulb anymore. So even though I have more bitcoin than when I started, I can't buy as many bulbs, and as I said, I like tulips.