It's to help stop any link back to where you purchased your coins to keep yourself secure, Blockchain Shared Send - https://blockchain.info/wallet/send-shared That's fine if you're sending an amount under 0.3 BTC to your Silk Road account. The more secure way is to use BitcoinFog, BitcoinFog - http://fogcore5n3ov3tui.onion You need more than 0.3 BTC to use their service.