What is the purpose of a spaced cross-hatch of grooves on the inside of a cast iron pan?
This kind of pattern seems to be added to a pan to increase its cooking capabilities.  This advertisement here suggests that the pattern itself is useful for making "crispy potatoes or a juicy steak", though it doesn't happen to say why the scoring helps, it does mean some people believe it will improve the food cooked in the pan.  

It is possible that the lines give some texture to the sides of the pan, letting pieces of food be pushed up on the sides for a while without sliding back down, and so spreading the food out and giving better control over access to direct heat for browning, or letting pieces rest away from direct heat after they're done or until there's room for them.  Sort of similar to the theory of cooking in a wok, only not because woks depended on quick hot cooking, so smooth sides and falling back to the center were positives.

It is possible that when dealing with single larger pieces of food, the scoring gives a kind of access to the underside, letting oil or other liquids run underneath (and be wicked up by or rendered off of the food as needed) without disturbing the food before it is finished cooking - especially if there are reasons lifting or flipping shouldn't be done too often, if it needs the uninterrupted cooking time to hold something delicate together or form a crispy crust.  

It is also possible that one of the reasons might be the pattern of grill marks or patterns of the scores left on food cooked in it (aesthetics does matter to some people), or that somebody had a theory as to why the score marks let the pan cook better, and so the pan is made whether or not it actually does cook better.  It is possible it had to do with visible effort - that this kind of pan was intended as art or display as much as cooking pan, and the extra difficulty cleaning it was secondary to that.