If Bitcoins aren't tied to a conventional currency, how did they get so valuable?
They are not "tied" to it but there are a lot of people who need to use fiat currency in order to buy and sell bitcoin, there are not enough places that accepts bitcoin so you can live with it forever, if every shop and store you buy things and all cafes you eat and all taxis fares you take were capable of accepting bitcoin than you wouldn't need to change your bitcoin to fiat currency but unfortunately that is not there yet.
Hence, this is the reason why people calculate the success of bitcoin with fiat currency, not really comparing the price of fiat and bitcoin but basically comparing the purchasing power of bitcoin and fiat. It doesn't mean it is "tied" but it means it is compatible, the comparison is the reason why we know how much bitcoin can buy in the real world right now.
We can't say "I purchased a bread for 150 satoshis today" yet but we can say "I purchased a bread for 1 dollar which is roughly 150 satoshi" which is the reason why it exists.