What are all the uses for PTR records / Reverse DNS in the context of web hosting?

Question

Are PTR records used by common web hosting services like: smtp, pop, imap, dns resolution, http, https, ftp, ssl and tls?

Answer 1

From the Wikipedia page:

The most common uses of the reverse DNS include:

- The original use of the rDNS: network troubleshooting via tools such as
  traceroute, ping, and the "Received:" trace header field for SMTP e-mail,
  web sites tracking users (especially on Internet forums), etc.
- One e-mail anti-spam technique: checking the domain names in the rDNS to
  see if they are likely from dialup users, or dynamically assigned
  addresses unlikely to be used by legitimate mail servers. Owners of such
  IP addresses typically assign them generic rDNS names such as
  "1-2-3-4-dynamic-ip.example.com." Some anti-spam filters assume that
  email that originates from such addresses is likely to be spam, and may
  refuse connection.
- A forward-confirmed reverse DNS (FCrDNS) verification can create a form
  of authentication showing a valid relationship between the owner of a
  domain name and the owner of the server that has been given an IP
  address. While not very thorough, this validation is strong enough to
  often be used for whitelisting purposes, mainly because spammers and
  phishers usually can't pass verification for it when they use zombie
  computers to forge domains.
- System logging or monitoring tools often receive entries with the
  relevant devices specified only by IP addresses. To provide more
  human-usable data, these programs often perform a reverse lookup before
  writing the log, thus writing a name rather than the IP address.

Answer 2

common use for looking up a PTR record is done by spam filters. Concept behind this idea is that fly by night spammers who send e-mails out using fake domains generally will not have the appropriate reverse PTR setup at the ISP DNS zone. This criterion is used spam filters to detect spam. If your domain does not have an appropriate reverse PTR record setup then chances are most e-mail spam filtering software will block e-mails from your mail server.