The hope with the TPM is that the system boots to a standard login screen, and the thief doesn't know any user's password. Much like someone snatching a laptop that's in 'suspend' mode.
Of course, a thief could try to bypass the login screen by e.g. booting with a different kernel command line, or a different initramfs. If you want to avoid this vulnerability, TPM unlock can be configured as a very fragile house of cards - the tiniest change and it falls down. The jargon for this is "binding to PCRs"
The fallback is you have to manually unlock the drive, the same as you did without a TPM. But the benefit is while things remain unchanged, the system can reboot itself.