I'm in country with a cash economy, am unable to use my debit card in the local ATMs and don't speak the local language. I login to online banking on my phone or in a cybercafe and send a bank transfer in my currency from my account to WU. Within a few minutes WU confirms receipt of the funds. I walk down the street to the large WU sign, show a MTCN number and my passport to the teller, and collect equivalent cash in local currency, less a bearable-in-the-circumstances transaction fee. (This actually happened by the way.)
Let's for the sake of argument, assume I already have a large fortune in Bitcoins, access on all the major exchanges and private keys memorised and written down in a secure box, all in my possession. Trouble is vendors want local currency not BTC, and I don't have the language skills to convince them that actually BTC is a much better store of value or locate the local BTC enthusiast's group (if there is such a thing) and negotiate an informal money transfer with them. In the end I probably give up and use Western Union...