From http://www.ut99.org/viewtopic.php?f=15&t=4935&p=52332

The usage of a random seed is well known for games that utilize syncronization as their main networking method.
While it is subject to desyncronization in different/buggy hardware conditions, it can be used with great advantage even in games that use replication as their networking mode.

The theory consists on randomly generating the base seed and sending it, then let both server and clients do the processing separately, expecting the same results.
This random seed gets some operations applied onto itself and if the hardware instructions on both ends do the same job, then the random seed allows the 2 systems to produce the same results without the need of extra communication.

Application: Siege Fragmentation Spawner.
When a fragmentation grenade explodes, instead of spawning the fragments, it will spawn a single frag spawner (bAlwaysRelevant, to all clients) which happens to also be bNetTemporary (replicated and then channel closed).
The fragmentation spawner will generate the base seed on the server, and will replicate it to the client counterpart.
Once done, it will proceed to spawn the flak chunks in all directions with the same results on both ends.
The fragmentation actor stays alive for 1 second to allow slow or lagged clients to receive said actor during that time.

The impact isn't as easily noticeable when you either insert one, or spawn a low amount of fragments in a net-game, but if you increase the numbers (5 spawners per sec, 100 chunks each) then you'll be saving a lot of bandwidth by simulating the chunks instead of replicating them.