You don't have to convert it back if you don't want to.
But converting it "back" to dollars/fiat allows a simple way of measuring your investment on any given day.
It depends though on how you want to measure your BTC holdings worth. If you have a value for stones say - you can always measure yours in BTC/STONES vs others who like to use BTC/FIAT. Others may prefer to see the worth of BTC/PIZZA who knows.
Either way - comparing the value of one item to its comparable value of another doesn't lower eithers worth. It just shows the worth of each at the price the market is willing to pay today.