It's not about "better". But if someone offered me a product for 100 dollars, and someone else offered me the same product for 100 euros, then I'd certainly prefer the 100 dollar offer. And to decide that, I have to know the absolute value of the euro in dollars.
Now it's unlikely that I get a product offered both in euros and dollars, but most probably a product I get offered in bitcoins is also available in euros (I'm in Europe; in the US, you'll likely get it offered in dollars). And if I don't know the absolute value of the bitcoins in euros, I have no idea which of the offers is better. Therefore the absolute value matters for anyone paying in bitcoins.
Why anyone speculating with them needs to know the absolute value should need no explanation.