Inverse domain is used to map an address to a name. 

For instance, if a server receives a request from a client 
and this server has only the ip addresses of the clients in 
its list then the server needs to find out if this client 
is on its authorized client list.

In order to determine if the client is on the authorized 
client list,server asks its resolver to query to the DNS 
server to map an address to name. 

And this type of querys are called inverse query(pointer 
query -PTR).