When they are actually useful
Negative slots matter most when you have something real to run.
That could be an EV charge, battery charge, immersion heater, dishwasher, washing machine, or another flexible load. A negative slot with nothing useful to move into it is interesting, but it may not be worth acting on.
Why the whole day still matters
A negative half-hour does not cancel out careless usage during expensive periods.
A day can have one or two excellent slots and still have a costly evening peak.
Why alerts help
Negative prices do not happen every day. Checking manually gets repetitive.
A negative-price alert is useful because it only interrupts you when the condition actually happens for your selected region.
Practical example
A short negative period might be enough to start a dishwasher or top up a battery. It may not be long enough for a full EV charge.
The useful question is not just “how low did the price go?” It is “is the cheap period long enough for something I actually want to run?”