what is the purpose of lemon/acid in fruit maceration for pie baking? A lot of pie recipes will tell you to take out the liquid in juicy fruits, like apple and peaches, for pie baking. 

The purpose is to avoid a soggy crust with too much liquid. 

Some people even put tapioca starch to thicken the liquid. 

Fruits must be macerated to take out the liquid. This is done by slicing and then adding sugar but also lemon juice. 

But what is the purpose of the lemon juice? I know that it lowers the PH and increases the acidity. 

But how does this assist in taking out the liquid from the fruit (I"m assuming this is the purpose). 

Why don't  just skip it?

I googled it but the answers didn't seem scientific or accurate? E.g., lemon juice is to add flavor or it is to speed up the process (but how?)
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).