Why does InnoDB need Mini Transaction (MTR)?

I don't see some other DBMS, like PostgreSQL, has the same concept. Per my understanding, a transaction is a set of log records. But MTR is also a set of log records. So why does InnoDB need transaction->MTR->log records this three-levels structure rather than transaction->log records two-levels structure? What's the benefit or what's the consideration of the design?