How is this different from a risk assessment?
A risk assessment covers the task in general and is written in advance. A pre-start check tests whether today's actual conditions match what the assessment assumed — the weather, the site, the kit and the people who turned up.
Why record a stop-work reason?
Because stopping work is the correct outcome sometimes, and a business that can evidence it stopped work for a documented reason is in a very different position from one that cannot show why a job was delayed.
Does every job need one?
That is your call, and it usually depends on the risk profile of the work. Many businesses use them for site work, work at height and anything weather-dependent, and skip them for routine domestic callouts.
Can it be completed offline?
Yes. Pre-start checks happen at the start of the day on site, which is exactly where signal is least reliable, so they are captured on the device and synced later.