Quick answer: Network bugs that 'happen sometimes' are hard to reproduce. A deterministic replay system that captures network state lets you replay the exact bug from a player's session.
Network bugs without replays are guesswork. With replays, the bug becomes a unit test.
Capture network state
Record all incoming network messages with timestamps. Replay buffer hold last 30 seconds.
Player upload on report
Bug report includes the replay buffer. Engineers reproduce the network sequence locally.
Determinism is mandatory
The game must produce the same state from the same network input. Floating-point determinism, RNG seeding, all of it. Otherwise the replay diverges.
Replay as test case
Once you fix the bug, the replay becomes a regression test. Bug shouldn't reproduce on the fix.
“Deterministic replay is one of the highest-leverage investments for multiplayer games.”
If you can't afford to make your entire game deterministic, make the network layer deterministic. Recording network sequences for replay is the lower bar; gameplay determinism is the higher.