A one-time signature (Lamport, Winternitz) reveals hash preimages chosen by the message, so each key may sign once. Merkle trees authenticate many one-time public keys under one root: stateful schemes (LMS, HSS, XMSS, XMSS^MT) use each leaf once and must persist which leaves are used; SLH-DSA picks leaves pseudorandomly and adds a few-time layer (FORS) so no state is needed, at the price of much larger signatures.
Their assumptions are the most conservative of all post-quantum options, so they are favored for long-lived roots of trust: CNSA 2.0 requires LMS or XMSS for software and firmware signing. The risk with stateful schemes is operational: restoring a key from a backup, or cloning it across HSMs, can reuse a one-time key and break security.