Deburr carefully ?

I'm soon to be entered in the 'black death society'. But before that I have some small tasks remaining on the fuel tank preparation.

I just noticed on DWG16A (fuel tank plans) that on the top view there is stated "drill #19, deburr carefully !" on the skin to spar attachment holes

I was wondering why they are so explicit on the "deburr carefully".

Why is that ? Does it have to do with possible cracking of the hole while dimpling this larger size hole for the 8R8 screw ?

Another question.
These holes are drilled when the skin is off the spar. Is there any risk of misalignment or will I just be fine by enlarging the pre drilled holes to #19.

You are correct, the larger holes seem to be more prone to cracking when dimpling. As far as potential misalignment, the holes are pretty good and if you use a piloted bit, there won't be any problem. You'll probably be fine even with a normal bit.