It’s a specific sound alarm so you know you’re low.
Because you can’t check your phone when you’re driving it’s a legal requirement to check your levels are above 5 before you start your journey and if it alarms anyway you legally have to pull over—it doesn’t matter what the exact reading is, if it sounds you’re too low to drive, you stop immediately and treat.
However, this wouldn’t work in a theatre setting as you can’t have the alarms on.
it’s also not always a simple case of I’ve alarmed I need to treat, it’s more of a decision process about ‘what’s my exact number, how quickly is it dropping, when did I last eat, what’s the glycemic index of what I last ate, how much insulin do I have on board’.
Some diabetics can physically feel when they are low, but a lot can’t and are 100% dependent on sensor readings as they’d go from feeling fine to in a coma within minutes.
And before people chime in with ‘what about before phones’, before phones T1 diabetics treatment involved holding blood sugars at a much higher level to avoid going too low—this was actually easier care wise, but long term health prospects (and cost to NHS) was much higher. So not being able to ‘disconnect’ is a small price to pay for keeping health, eyesight, and all limbs.