Not just aesthetic. My experience on every bike I've ever built is this. For the rear the housing runs forward horizontally from the TT on one side or the other of the stem or HT. It also has some amount of its own weight. The combination of its weight and the stiffness coming off the TT make it want to drop before somehow going high enough to enter the lever from above. If it isn't supported it ends up with a bend where it enters the lever, essentially trying to enter the lever from the inside. The longer you make the housing to open up that bend, the heavier it is so the more it wants to sag. It may not look that way in ad photos where the steering is pointed straight ahead, but when you turn the handlebar toward whichever side the lever is mounted it falls down and kinks the cable at the lever. When routed above the bar it rests against the back of the bar. This give it a slightly sharper bend upward off the TT but this bend is still less than the kink it can develop at the lever. You can experiment with whether it should go on past the stem on the side with the lever or on the other side. I can't remember which way I do mine, and it sort of depends on how you run the front and whether the TT has braze-on housing eyelets on one side or the other.

On my bikes the handlebar always sits ahead of the front of the fork. So for sidepull brakes routing the front cable behind the bar means it drops straight down to the caliper. By choosing whether you run it across the rear cable you can make it help keep the rear cable in place too. FWIW, I always have the front brake on my left hand, and my sidepulls have the cable on the right, so the cable always has a smooth loop over the top, left to right. For centerpull brakes the hanger tends to be directly below the stem extension but still behind the bar. Looping the cable over the stem to enter the hanger from the opposite side gives it a nice smooth bend.