When you have the wheel laced, but not tensioned, all the heads-in spokes will be pointing out of the wheel, then bending back in to the rim. It looks almost puffy. When you tension a wheel in this state, you're building in the extra stress from that bend, and that will accelerate fatigue at the j-bends.

To prevent this, you take your laced, untensioned wheel, and push all the spokes flat as they come out of the hub flange. This adds to the bend coming out of the flange hole and gets the spoke naturally toward the rim hole.

Heads-out spokes don't usually have a problem with this, but NDS MTB rear hub spokes can sometimes require a bit of bend in the opposite direction (opening the j-bend a bit).