Safest way to trigger camera shutter using arduino
Optocouplers can provide galvanic isolation between your Arduino based circuitry and the camera electronics. This means that certain dangerous effects wont pass between these two systems. An electrostatic discharge or a voltage spike from a switching supply can only damage one side, if the isolation was properly implemented.
Field-bound interference e.g. emitted from a malfunctioning BLE module wont be affected and could in turn effect the camera.

As an exampel of an actual product, the cheap Yongnuo RF-603 remotes are capable of triggering a camera. Their electronics appear to use no optocoupler at all.
A proper camera trigger is unlikely to damage the camera. A tinkerer could however implement a bad trigger possibly feeding high voltages or curents into the camera. Optocouplers can be misused to minimize the damage inflicted by such a circuit.
This looks different if you want to trigger old flashes remotely. Their high voltages on the trigger pins can actually damage your remote or camera.