The Kerberos key distribution center (KDC) uses this account. "Krbtgt" is part of the KDC's service principal name (SPN). Also, a symmetric key is derived from the password of krbtgt, and this key is used to encrypt and decrypt TGTs. Only the KDC knows this password and it changes the password periodically.
This account is used in Win2K & greater to issue a TGT. This account is disabled by default and is need for the Kerberos Authentication Protocol. This ia built account and is created during the dcpromo process (when you raise your machine into a Domain Controller) on a DC
The following KB articles would give more information on this question