Buy bitcoin at $1 on Jan 1, sell at $10 on dec 31. I now owe taxes on $9 in gains, so I’ll need USD$3 on April 15. If on Jan 2, I buy more Bitcoin at $10 and it drops to $6 on April 15 that will suck because then I’ll need to sell half my Bitcoin to pay taxes.
I also need water to live my life, but that doesn't make me feel the need to price everything in gallons of water.