Presumably the owners of these signs feel they are less likely to be rammed by another driver.
I don't know about other people but I try to avoid other cars when I am driving. If it should happen that I am about to crash then one of these signs will not enable me to bend the laws of physics so as to avoid a car which was otherwise unavoidable.
To further confuse the issue, most cars with a 'baby on board' sign do not have a baby on board; I mean, surely having a baby on board is a basic requirement to be able to display a 'baby on board' sign?!