Essential information first: None of the reasons I give have anything to do with Valheim's game mechanics. At least as far as I know. Wall sections are not the only structural items that have reinforcing horizontal cross members. Stake wall sections have them also.

In real life, if one erected a stake wall with the horizontal cross members on the outside, one would be providing a finger hold and/or a toehold for an enemy scaling the wall. Therefore, I erect stake walls with the cross members on the inside.

In real life, it one erected an exterior wooden wall with the horizontal cross members on the outside, one would be providing horizontal spaces where rainwater could collect rather than run off. This could easily lead to faster weathering and weakening of said cross members. Therefore, I erect wooden walls with the cross members on the inside.

Players are free to object on aesthetic grounds. If a structural item performs its intended function, I don't give a rat's rear end how it looks. Anyone who does care is free to erect walls with the horizontal cross members facing whichever way they want.

To repeat, as far as I know, it makes no difference in terms of game mechanics.