Grounding of a generator is required for two reasons. Firstly you will need to gain a reference for your voltage. You may get 380v between phases and 220v to neutral but if the neutral starts to float due to ground surges or induced voltages with in the ground, your voltage to earth may rise above the insulation levels of your system, especially electronic items. This can happen normally but extremes when lightning is around. Secondly, there will be no earth fault path so the first earth fault will not cause the over current protection to trip. The second may cause some reaction but still not a trip. If one of them is a high impedance fault the current may be extremely low until you touch the metal of the faulted part. Your resistance is lower and you are then killed.
Earthing is not bonding. You must still bond your portable generator to the earth system of the installation.
Some supply authorities have regulations that prohibit an independent power supply for being connected to the power supply earth as they do not wish the IPP (Independent Power Producer) to inject earth faults into their network.
I have had 1st hand experience why there are 2 "grounds" on a portable generator. We were camping in a trailer on the beach in California and the generator was about 15 feet away on the sand. It was providing 240/120 volts AC power. The Santa Ana winds started blowing after about 3 days, which blow from the desert out to the sea and are very electrically charged. After about 4 days, the 12VDC lights would not work, and the smell of smoke was coming out of the load box. My 12VDC rectifier had burnt up.
After investigating it a little, I found that the equipment ground in the generator supply hookup is separate and isolated from the one on the face. My theory is that the frame of the generator and the frame of the trailer were building a static charge as they were insulated from each other on the sand. If there had ever been a ground fault, then the circuit breaker would trip, but I should have earthed both devices and connected a separate ground conductor between them Essentially, the static discharge path was from the trailer frame (negative is chassis grounded) through the rectifier into the neutral and back to the center of the 120/240 VAC generator. I have since then, always used an additional ground/earthing conductor between the generator, trailer, and nearest metallic water hookup pipe.