Hint: I'm an ex-Marten maintainer, so the similarity is not accidental ;)
As Op said, not needing to rewrite applications or using the muscle memory from using Mongo is beneficial. I'm not planning to be strict and support only MongoDB API; I will extend it when needed (e.g. to support raw SQL or JSON Path). But I plan to keep shim with compliant API for the above reasons.
MongoDB API has its quirks but is also pretty powerful and widely used.