what are Null routes used for when configuring bgp in Cisco routers?

Question

ip route 10.100.100.0 255.255.255.0 Null0

Answer 1

Routes to Null0 are used in general (not just in BGP) to prevent routing loops when addresses are being summarized. When route summarization takes place, the router could get traffic for any subnet within the summarized range, and not all subnet ranges might be in use, so the default routes could loop the packets in a count-to-infinity scenario. Here is a reference link with a configuration example:
http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml

Answer 2

Ip Route Null0