What are the advantages and disadvantages of Merkle signatures over Lamport signatures? I know they are both hash-based and post-quantum secure, and that Lamport signatures can form the basis for a Merkle scheme, but what do you actually gain over standard Lamport signatures by doing this? To put it another way, since Merkle signatures make use of another one-time signature scheme, why would someone use Merkle signatures when they have a perfectly fine one-time signature already?
why would someone use Merkle signatures when they have a perfectly fine one-time signature already?


Because Merkle signatures are not one-time.  Instead, you can use a private key to sign a number of messages. There is a finite limit, however that limit is potentially large.