It's common sense. To see if you agree, try it yourself:
1. You have 378 worthless paper dollars. You can spend them today on a shiny new Bitcoin,* or wait until next year, when you could buy 1/2 of a Bitcoin with your 378 worthless paper dollars because inflation.
What would you do?
2. You have one shiny new Bitcoin, and, for reasons known only to statist pigs and other Keynesian, you wish to invest it in USD. You can buy 378 dollars with it today, or wait until next year, when you could buy 756, because dollar inflation.
What would you do?
*This is a perfect, hypothetical Bitcoin, it's buying power will never go down.
It is not applicable always.
You have 500 worthless paper dollars. You can spend them today on an i-Phone. Or you could wait till next year and buy it a lot cheaper.
People still choose to buy i-phones because there see utility value in the one year that they hold an i-Phone. Similarly, even in a deflationary environment people will continue to make purchases if they see value in what they buy.