from bitcoins wiki page:
"
Mining is the process of adding transaction records to Bitcoin's public ledger of past transactions. This ledger of past transactions is called the
block chain as it is a chain of
blocks. The block chain serves to
confirm transactions to the rest of the network as having taken place. Bitcoin nodes use the block chain to distinguish legitimate Bitcoin transactions from attempts to re-spend coins that have already been spent elsewhere."
basically, it uses your gpu to do intensive math equations required to make these "blocks". amd gpus, specifically the large gcn based 7900 series and the r9 series, are very good at this. even the 7870 could do more bitcoin mining than a geforce TITAN. heck, even the 7850 mined the same as a geforce 780 oc edition. anything in the 7900 or r9 290 series was much more capable.
this mining is listed in kilohash a second, or Kh/s. the geforce 780 hits roughly 300-320. the titan hits 360-400. the 7850 hits 380-440, and the r9 290x hits up to 1002.
when the r9 290 and 290x came out, bitcoin miners bought them in droves to power new bitcoin machines. merchants, seeing that these miners tended to spend ludicrous amounts chasing bitcoin, amped up the prices on high end AMD 7000 and r9 gpus.
until the craze dies down, amd prices will remain inflated. at the inflated prices, these gpus are a terrible value, since the are now more expensive than the 780ti, while offering similar performance.
here is a link to a pretty good explanation of bitcoin mining, makes some good reading material:
https://en.bitcoin.it/wiki/Mining
and here is a list of the Kh/s rating of most modern gpus, if you are interested in seeing exactly how much better amd chips are at it.
https://litecoin.info/Mining_hardware_comparison#AMD_.28ATI.29