In general, timestamps are used in various authentication protocols to help prevent replay attacks, where an attacker can reuse an authentication token he was able to steal (e.g. by sniffing the network).
Kerberos authentication does exactly this, for instance. In the version of Kerberos used in Windows, the default tolerance is 5 minutes.
This is also used by various one-time password protocols used for two-factor authentication such as Google Authenticator, RSA SecurID, etc. In these cases the tolerance is usually around 30-60 seconds.
Without the time being in sync between client and server, it would not be possible to complete authentication. (This restriction is removed in the newest versions of MIT Kerberos, by having the requester and KDC determine the offset between their clocks during authentication, but these changes occurred after Windows Server 2012 R2 and it will be a while before you see it in a Windows version. But some implementations of 2FA will probably always need synchronized clocks.)
Having clocks in sync makes it easier to work with disparate systems. For instance, correlating log entries from multiple servers is much easier if all systems have the same time. In these cases you can usually work with a tolerance of 1 second, which NTP will provide, but ideally you want the times to be as closely synchronized as you can afford. PTP, which provides much tighter tolerances, can be much more expensive to implement.