Does an acid actually stabilize meringue and if so why?

Acids allow more air to be beaten into a meringue. In order to make meringue, the proteins in egg white must be denatured. In their natural state, the proteins are curled up into tightly packed balls. When the egg is beaten, they uncoil into long strands.

These strands then begin to coagulate, or join together, with the help of the sugar you add. The air you whisk in gets trapped between these joining strands, giving the meringue its characteristic light texture.

Acid delays coagulation, which means that there is more time for air to get trapped in amongst the proteins, resulting in a lighter meringue.

The acids usually added to meringue are white wine vinegar, lemon juice, or cream of tartar. Fresh eggs are more acidic than old ones, so these help too.

Some cooks use copper bowls to make meringue, because copper ions from the bowl bind to a particular protein (conalbumin) and strengthen it.

I have made two separate meringue mixtures side by side: one with vinegar and one without. In my experience it makes no difference to the final outcome provided that you add the sugar really slowly (a tablespoon at a time) and not too early. If this is done correctly then there is no need to add an acid.