What Mode $06 actually contains
Mode $06 is the raw result of every non-continuous monitor the PCM runs. It shows the last catalyst efficiency test result, the last EVAP small-leak test result, the last EGR flow test, and dozens of other test IDs with pass/fail thresholds. Codes only set when a Mode $06 test fails a defined number of times. The data itself shows you a failure trending long before a DTC lights the dash.
Using it for driveability that has no code
Customer complains of a rough cold start but no MIL. Pull Mode $06 misfire counters. If you see 20 misfire events on cylinder 4 during the first two minutes after startup, you have the smoking gun without ever setting a P030X. That is the value of the mode.
Reading manufacturer-specific test IDs
Generic Mode $06 gives you standard SAE test IDs. Enhanced scan tools translate manufacturer-specific IDs into English (Catalyst Bank 1 Efficiency, EVAP 0.020" Leak Test, and so on). Learn to read both. It sets you apart from every parts-changer in the market.