Often in macro pure exchange economy models (sequential trading), authors make the initial period good price $P_0=1$ asserting this set-up is to make the consumption at period $0$ the numeraire.
But I don't see any detailed explanation or justification for doing this.
Just to add some background story:
The usual set-up is that agents are endowed with some units of goods, say oranges. So the unit of their consumptions is oranges. $P_t$ often the price, at $t=0$, of an orange consumption promised for delivery in period $t$. Why make $P_0=1$?