The TPM is used in Pureboot, for the computer to authenticate itself to the LibremKey.
We have worked with Nitrokey to add a custom feature to our Librem Key firmware specifically for Heads. This custom firmware along with a userspace application allows us to store the shared secret from the TPM on the Librem Key instead of on a phone app. Then when Heads boots, if the BIOS hasn’t been tampered with the TPM will unlock its copy of the shared secret, and Heads will send the 6-digit code over to the Librem Key. If the code matches what the Librem Key itself generated, it flashes a green light. If the codes don’t match, it flashes a red light.
From https://puri.sm/posts/introducing-the-librem-key/
I do not believe LUKS uses the TPM at this time or at least in the default configurations that I have seen.