Two servers each write a record stamped with its own clock. Why can you not use those timestamps to decide which write happened first, and what do you use instead?

Short answer

Wall clocks on separate machines disagree by an unbounded amount and can jump backwards, so their timestamps order events by accident rather than by causality. Logical and vector clocks capture what preceded what, quorums give overlap not order, and a lease is safe only with a fencing token.

Logical clocks are useful because they record causal order without pretending there is one global time. Good logical clocks reasoning explains when Lamport clocks are enough and when vector clocks are needed. Weak logical clocks answers sort timestamps and miss concurrent events.

Logical clocks give the system a way to reason about before and after without inventing perfect time.