I assume you mean ‘as well as’ rather than something being physically on top of something else. Not much difference, adding ‘all’ suggests there is more that you are adding to. I crashed my car, I lost my job, my wife left me and on top of all that my dog died.

The meaning is essentially the same = in addition to that. The second sentence could refer to one or more events preceding it. Adding 'all' emphasizes the fact that several things occurred first.