This allows devices in the receiving network to know where it came from, allowing a reply to be routed back (amongst other things), except when IP addresses are used through a proxy or a spoofed IP address, which does not pinpoint a specific user within that pool of users.
Network ingress filtering makes it much easier to track denial-of-service attacks to their source(s) so they can be fixed.
[3] Network ingress filtering is a good neighbor policy that relies on cooperation between ISPs for their mutual benefit.
As of 2012, one report suggests that, contrary to general opinion about the lack of BCP 38 deployment, some 80% of the Internet (by various measures) were already applying anti-spoofing packet filtering in their networks.
[6] At least one computer security expert is in favor of passing a law requiring 100% of all ISPs to implement network ingress filtering as defined in IETF BCP 38.