[WhyCause]

I have had iOS tell me I needed to "unlock the phone to enable accessories" (or something to that effect) when plugging in the charger that came in the box with the phone.

This has usually been when the lightning port has been fouled by pocket lint or the cord has started going bad, but I bet it happens often enough to desensitize users (especially for people using off-brand cords) that it has been identified as a possible attack vector, and developer mode is the solution.

[gruez]

No, the alert you see is different from the "unlock the phone to enable accessories" notification. That goes away (and your phone starts charging) as soon as you unlock your phone. The "trust this computer" is an alert that only shows up after you unlock your phone, and it looks like this: https://media.idownloadblog.com/wp-content/uploads/2018/02/i...