what is the purpose of lemon/acid in fruit maceration for pie baking?
While there might be an acceleration effect, sugar on its own is very capable of drawing out liquids quickly. Two other effects are probably much more important:


lemon juice is very effective in stopping cut fruit from oxidizing/enzymatic browning.
taste balancing. Fruit is naturally sweet and sour, adding heavy amounts of sugar can upset that balance too far into the sweet region, resulting in a cloying/one dimensional/stodgy character
in some recipes, keeping the sugar/fruit mixture from caramelizing/browning too much. Acid inhibits maillard reactions to a degree.
acid promotes sugar inversion when combined with heat, which slightly changes the sweetness profile (halfway to honey, more or less) and potentially significantly the texture (there will be no crystallization of loose sugar-water mixtures upon cooling).