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[HELP] ip route 0.0.0.0 0.0.0.0 Null0 255

The purpose depends on how the network design is. Some people use it to propagate default route over routing protocol such as BGP and OSPF to simplify routing table of neighboring routers. For those BGP connectivity to ISP where you wish to only receive default route, the ISP may use such static route to propagate the default route in addition to the full BGP table the ISP router has.

There is also network stability consideration in terms of default gateway propagation over routing protocol. As illustration, let's say there are networks that have multiple default gateway routes and use let's say RIP or OSPF to propagate default route. With RIP or OSPF on Cisco IOS you only use one command of default-information originate to propagate the default route. RIP or OSPF then will pick one default route with lowest administrative distance.

During the production network course, these multiple default gateway routes may fluctuate due to connectivity issue. You as network administrator wish to have stable RIP or OSPF network where the routing protocol keeps propagating default route to the rest of the network with minimal routing calculation. When you have such default route pointing to Null interface with the highest administrative distance, the routing protocol keeps propagating default route to the rest of the network even though all other default routes are gone, hence keeping the routing protocol routing calculation down to minimal.