Whether the trailer is connected to or separated from the earth by tires
or stabilizers is not the point. The primary purpose of the ground wire & the bonding of all conductive parts of a system is to insure there is a low impedance path that will open the source circuit breaker of fuse when there is a fault to any conductive part of the system.
If you do not bond the frame of a trailer to the ground wire, a fault (connection) between a hot wire & the frame will put the frame at full line potential (120v). Since the utilities use connections to the earth (grounding) for a number of good reasons, there will be a potential difference between the hot frame & the earth. Since dirt isn't all that good a conductor, even with your stabilizers touching the earth there may not be enough current flow to open the source fuse or circuit breaker. Touching the frame (or anything bonded to it) and the earth (through your shoes, etc) in an ungrounded system will cause a shock.
Again, in a properly grounded & bonded system, the low impedance ground wiring will cause the fuse or breaker to open, protecting the user.
As to why utilities make an earth bond, there are many practical & safety reasons that make sense in a large distribution system. If there was no utility earth bond there would be no shock hazard to the earth, but there would be far greater problems caused by transformer failures, lightning strikes, etc. This is why small portable generators do not have to be "earthed", and are actually safer when not. But, since the utilities & service entrances of most homes & campgrounds are required to make an earth bond, not bonding the frame, gas lines, and all metal parts of a trailer does not meet code, and is unsafe.
An interesting PDF covering some of the advantages & disadvantages of grounding & earthing of generators is at This Link