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.