Great, you listened to all the guys preaching deliberate practice, and are heading to a parking lot (or big empty field if you're a dirtbike) to do some drills! You get there and ride around in circles. How do you tell if you're getting better? how do you know you're making turns and not missing them by a mile? How do you tell if you went tighter this time than last? Without constraints or reference points you're just cruising around and can't tell what's working and what isn't.
By adding constraints (whether it's lines on the pavement, cones, tennis balls, your backpack, or SOMETHING) you're giving yourself a reference point. Put that backpack at the end of a "turn" and see how close you're getting to it. Or put the bag in the center of your slow turns exercise. Give yourself more information so that you can compare more accurately between attempts at a drill. Whether it's a way to tell if you're at a consistent distance doing circles, or if you're running wide at turn exits, you need an easy reference point to measure from. Otherwise you're not controlling for variables. Since training is a form of experimentation, you want to keep as many variables as possible static and only change one thing at a time.
An added bonus is that throwing markers down gets you reference points to look at. whether it's turn-in, apex, exit, or "i dont want to hit that" kind of markers, this lets your exercises be more deliberate, more repeatable, and measurable.