IIRC: it is to leverage the OS page cache rather than having a separate buffer pool in user land. By default lmdb uses normal pwrite/fsync for the write path, but can optionally use a writable mapping and (presumably) msync.

However, some people think there are problems with this usage: (pdf warning) https://www.cidrdb.org/cidr2022/papers/p13-crotty.pdf