There is good reason for that. A 'precautionary landing' is one which there is a good likelihood that everything will be fine on landing and an evacuation will be unlikely, but what it does is get the cabin crew prepared for their drills should the worst situation occur, I'm not cabin crew, but from memory think it is along the lines of silently identifying your able bodied passengers to assist in an evacuation, and identifying where people with disabilities or people who may impede an evacuation are etc.

An 'emergancy landing' is one where the outcome is unknown and will more than likely involve an evacuation with potential severe sctructural damage to the a/c. Everyone is prepared and briefed, and able bodied pax are identified and asked to assist in evacuation etc, also the people sat in emergency exits I believe are asked again if they are prepared to operate the exit in the emergency etc.

If there is any doubt then we get the cabin secured for a precautionary landing, rather than have a nasty shock once on the ground that catches everyone off their guard.

BF