Consider a scenario where you have a log file and in the log file is an IP address (172.217.5.14). You are not sure whose IP address it is. So you run the following commands:

# dig -x 172.217.5.14 +short

lga15s49-in-f14.1e100.net.

ord38s19-in-f14.1e100.net.

# dig lga15s49-in-f14.1e100.net. +short

172.217.5.14

# dig lga15s49-in-f14.1e100.net. +short

172.217.5.14

The first command (dig -x) checks the PTR record for the IP address 172.217.5.14. It returns two PTR records: lga15s49-in-f14.1e100.net. and ord38s19-in-f14.1e100.net.[0]. Those are subdomains of 1e100.net, which we know Google owns. However, you can set a PTR to pretty much whatever you want, so we now take an additional step as well. We run the dig command again to check the A records for the domains. This returns the same IP address we started with, which is good. Since Google controls the DNS for 1e100.net we can be reasonably sure that it is in fact a Google server. This is called Forward-confirmed reverse DNS (FCrDNS) and is one tool you can use to determine the ownership of an IP address. For example, it is frequently used as a weight in email spam filters. Although, because of the intricacies of email, in that case it is usually not used for identification and instead used as a general purpose check to determine whether a mail server is rogue or not, since spam servers very often do not have proper FCrDNS.

There are other tools to determine who owns an IP address, like whois, but in some instances one will garner useful information and the other will not. So it's nice to have both at your disposal.

[0] As a side note: the trailing . in those PTR records returned by dig is not a typo. All domains actually end in a dot, it's just usually implied.