Basically, all your drainlines slope downward, through all their connections, joints, and fittings until the whole system connects with the city sewer, usually under the street in front of your home. I like to compare it to a tree, with a fixture at the end of each branch, and the whole connected to a trunk. When the system works, as it usually does, it works invisibly.
Most people never give a thought to their drainlines.
They take it for granted that their toilets will flush away their waste, that their showers will drain, that their kitchen sinks will make the remains of their meals disappear. It is only when these conveniences stop that the drains are given any thought at all. Every drainline in your home has limitations. These extra branches are there only to allow drain-cleaning machines to run through and clear out blockages. This way, if your kitchen sink is stopped, we can run a larger, longer and stronger snake through the cleanout instead of through the sink drain.