Why blanch handcut fries?
When I was working as a cook, we did this to cook the potatoes enough so that we would have a better result after deep frying. Deep frying alone can make for a fry that is not always cooked through to the center. If you blanche, or par-boil, the potatoes ahead of time, some advantages are these.


Frying goes more quickly, because the potatoes are partially cooked
The fries are more evenly cooked, because you don’t have to rely on
the deep fryer to do all the cooking


Probably not an thorough list of advantages, but these are the biggies.

If you go to hand cut fires, make sure cut the size you go with is consistent. This make of a better deep frying experience.

You can also deep fry potatoes at a low temperature (300 degrees F) for some time as an alternative to blanching.