Phantom 3 series drones use brushless motors for propulsion. At full speed with sharp blades, especially the carbon fibre ones, they have enough force to chop yours or somebody else’s finger off. If your drone hits someone it can seriously injure the person. Having the propeller guards should at the very least mitigate the injury caused.
Safety issues aside, they could stop you from crashing and destroying your drone. Say your drone touches a wall. Now those high RPM propellers would break to pieces sending your drone to the ground. If it was high enough chances are you would need to get a new one or at the very least spend hundreds of £ replacing the gimbal, camera and possibly the shell.
With the guards attached it would simply bounce off and carry on flying.
I understand that the guards add to the total weight of the drone and mess a little with the airflow, shaving off valuable 10-30 seconds of the flight time. Now imagine what you could have done with all that time? Personally, I have never used more than 75% of the battery.
There are two types of guards available: permanent ones and detachable ones. There almost zero difference between the two and the obvious choice is to go for detachable ones.
They cost few quid and take about 10 minutes to install, only because you have to deal with 16 screws.
My personal view is that DJI should have included that accessory by default. I see no reason why you shouldn’t install them on your drone as they don’t affect the flying characteristics of the drone and encourage everyone to do so. I have mine the same as in the picture above, they are nothing special but they do the job.