It can be useful to quickly look up domain name of IP and find neighbour domains served by the same IP, also to find definitely busy (and probably spare) domain names.
Also it's value will jump high when the global DNS system suffer some catastrophic failure (e.g. access the rests of Internet using local DNS database, not just things recovered from cache).
Where to get it or how to quickly generate it? Scanning the whole 0.0.0.0/0 by some on-line reverse-DNS service is too suboptimal.
Of course it will almost always be a bit out of date, so the source of small daily deltas to it is also welcome.