Sometimes you want to restrict access to something to a specific set of devices - for instance, you might want your corporate VPN to only be reachable from devices owned by your company. You can't really trust a device that self attests to its identity, for instance by reporting its MAC address or serial number, for a couple of reasons:

• These aren't fixed - MAC addresses are trivially reprogrammable, and serial numbers are typically stored in reprogrammable flash at their most protected
  
• A malicious device could simply lie about them
  

If we want a high degree of confidence that the device we're talking to really is the device it claims to be, we need something that's much harder to spoof. For devices with a TPM this is the TPM itself. Every TPM has an Endorsement Key (EK) that's associated with a certificate that chains back to the TPM manufacturer. By verifying that certificate path and having the TPM prove that it's in posession of the privat