Parsec question

Let us just say that I don’t like meters, kilometers or miles and I would like to have a unit that represents a slightly larger distance. So I go out and select an arbitrary landscape feature, for example the Golden Gate Bridge and I place myself at such a distance from it that when I extend my arm and hold up my thumb and squint, my thumb will nicely fit between the two abutments of the bridge. Then, I say that this is the perfect distance I was looking for, since it is much, much more convenient to name this distance “BridgeThumb” than using “3.26 kilometers”, instead.

Dear astronomers, I have no education in astronomy whatsoever, and this is how “parsec” looks like from my perspective. I’m sure I’m missing something here. Can someone explain it to me why using “parsec” is justified?