A transaction gives atomicity, a lock gives serialization — they aren’t substitutes. How pessimistic and optimistic locking cover the Lost Update that an isolation level’s plain reads don’t, and how to choose between them.
A transaction isolation level is a policy choice about which anomalies to allow. The anomalies each ANSI level prevents, and how InnoDB’s MVCC and Repeatable Read behave differently from the standard.
What each of the four ACID properties actually guarantees in an RDB transaction. A/C/D are relatively clear guarantees, but only I has ’levels’ — the gateway to the correctness vs. concurrency trade-off.