TPMS give you the convenience of short passwords (or no passwords) and the security of long keys.

A chip which you can write to and interact with but can't read is valuable; it lets you enforce conditions which you otherwise couldn't. For example, you can protect your sensitive data with a 6-digit pin, secure in the knowledge that the chip will erase the encryption key after 10 failed attempts. If you had full access to the TPM storage, you could brute force that PIN in seconds.