Cisco turns on route synchronization by default. This is to assure that the networks BGP is advertising are in fact reachable. However, if the administrator on the receiving end of the Internet connection is using static routes on his LAN, it may be advisable to turn off synchronization at the Internet router, especially if their network is unstable and tends to crash or fail with any significant frequency. By turning off synchronization and redistributing static routes into BGP on the customer side of the Internet connection, the ISP sees the customer as "always up". ISP's actually use this mechanism after aggregating their routes to make sure major peering connections stay active.
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