vertical water supply direction

this may sound like an odd question... but i've noticed in most designs i've seen that water supply systems are designed so that if the pipe goes vertical, the water moves UP AND NOT DOWN...

is this a design pillar? because what i am doing is splitting a house into two units, basement and the main unit. i want to minimize, if not possible to eliminate ANY DEPENDENCY between the two, which includes the space between joists in the floor between, i.e. i want it to be utilized as much as possible by the basement and not the upstairs unit.

so once the pipe enters upstairs, i want it to never have to go back downstairs but it needs to go from one side of the house to the other, i.e. use the ceiling space to go on the other side and then descend. e.g. the pipe enters near the right wall, where the water heater is but then it needs to go across to where the washer is. but i don't want it to go under the floor but through the ceiling and then come down.

in a nutshell, is there any issues with water in a vertical pipe flowing in both directions instead of only going up, which is like everything i've ever seen?

thanks