Down and out is a tough situation for a human being, but the ONLY way to be for a temperature/pressure relief drain line. That’s because the T&P’s (red circle) job is to instantaneously vent pressure, and steam or water is a part of that.
If the line is plumbed uphill or through too many twists and turns on its way out, the venting may not be sufficient and the tank may explode in an emergency.
What’s more, if it opens once and water pools within the line, it will corrode the valve and prevent it from working at all.
This picture illustrates a common problem. There once was a taller tank here with a T&P that connected to a line that went over, down and out. When this tank was put in, the installer didn’t know any better, so instead of cutting a new hole in the wall, he simply ran the line over and up to meet the old drain line (yellow arrows). When it comes to water heaters, ignorance is not bliss.