Reading the document will give you more information.
But here is the gist for what I use it for:
Total outgoing download traffic for the Server/Relays
The Server can be configured to throttle the cumulative file downloads at any given time. When this throttling setting is enabled, the Server/Relays send out no more than the specified number of bytes per second for all file downloads (including IEM Clients and child IEM Relays). This setting is especially useful if there is a concern in a local area network that too much bandwidth will be used when a patch is sent out to many IEM Clients simultaneously.
BigFix Server cumulative download throttling can be configured by using the _BESRelay_HTTPServer_ThrottleKBPS setting on the Server.