Money is important, it provides liquidity for trading. It is based on trust, so it makes sense that it can feel arbitrary, but you simply have to compare a world where you must barter assets directly between those who actually want that specific asset to realize how much of an improvement money is; meaning its not so arbitrary.

What gives money its value? Well it depends on the money. Fraud, violence, trust, in the case of Bitcoin: cryptography and game theory.