To solve chicken and egg problem. In order to enter password, you need to boot full OS (with keyboard and display drivers, for example); but to decrypt OS boot

To solve chicken and egg problem. In order to enter password, you need to boot full OS (with keyboard and display drivers, for example); but to decrypt OS boot drive you need a password (or more precisely, some key material derived from the password). So a copy of that key material is stored in a TPM, and provided to software only if measured boot goes as usual (i.e. this is not a Linux booting from USB thumbdrive).