It's easy to poke fun at this, but maybe it's not so silly.
How much is a "point" worth? (What is the point scale?) If it's a 100-point scale, this might push somebody over the line by a half-grade (in our typical U.S. grading system).
If the parent gives blood as a result, it might mean that they are a good citizen looking out for the welfare of everyone, and that they are concerned about their child's future. This would seem positive for the child's education. If a child is teetering on the edge of some grade category or entrance requirement, then who's to say this isn't as valid as knowledge testing.
Well, it IS an entrance requirement, and so the intent must be to predict future results. So, it seems to have some relevance to me. Sure, it's a bit arbitrary and clearly tied to some unrelated state goal. But might be predictive of future results. The family goes along with the system and pitches-in to help, and so the child will likely do better in school.
It's not much different than considering non-grade aspects for entrance. Yes, the very idea of an entrance requirement for high school (other than making an appearance for n years) is foreign to westerners. But we also do have some tradition of giving some little "extra credit" or recognition for community participation - e.g. clubs and activities, etc. for entrance to college, or to some selective schools, etc. How is this that much different? Yea, it's about the parent, not the child, but I think it is seen as more of a family unit.