What are PTR records actually useful for and would one make PTR records for hosts using private IP addresses?

I was reading up about PTR records, as I was unsure about what the point of them were and came across this site, where there is an an example where there are a records made for hosts with private IP addresses.

I'm assuming that CloudDNS know more about networking than I do, so this is something that is done for some reason.

https://www.cloudns.net/wiki/article/40/

So, what are PTR records used for? Are they for authenticating email delivery source, like SRP and DKIM records?

Why does the example on this page have PTR records for hosts with private IP addresses pointing to x.0.168.192.in-addr.arpa?