Targeting, or location, is one of the many skillsets that a pitcher employs. Why? Because of the impacts of - (1.) pitcher's physical makeup, (2,) the pitcher's exposure to the competitive environments, and (3.) the pitcher's place in time.
(1.) The pitcher's physical makeup is primarily concerned with the flexibility of the human body and its ever changing state of being. The human body is a collection of joints, muscles, tendons, and weight distribution that makes every move, every pitch slightly different from the last pitch. Temperament, mental attitudes, and other personal dynamics has a definite impact on a pitcher's appearance. These subtle, yet important influences on a person, can sometimes mean showing up - but being somewhere else.
(2.) Exposure to the competitive environment requires understanding the changing nature of each batter faced, multiple play scenarios, and other such events.
(3.) Finally, a pitcher's place in time refers to three fundamental conditions: (a.) bullpen duty, (b.) game time, (c.) rest after an appearance. I deliberately did not mention off-season and rehab situations because those involve too many things that have relative importance.
(a.)So, location during bullpen duty gives a pitcher the opportunity to see how the repertoire is working that day. Taking all the things mentioned heretofore, the pitcher will usually "spot" his go-to location that's most familiar. The pitcher's mainstay pitch is usually used to hone in on the location that generates the best results. However, sometimes that location can change, slightly, because of what I mentioned above.
(b.) During an appearance, the pitcher usually fits into the mound, then takes from the bullpen as a reference point, and refines. Mound conditions, backstop relationships, umpire persuasions and such, all require a bit of tweaking to one thing or another. Sometimes a slight change in the body's motion like - a slightly long or shorter stride, a more aggressive drive forward, slight alteration in the shoulders, and so forth.
(c.) During days off can give a pitcher the chance to see how everything is getting ready for the next appearance. How does one feel overall? Any sores or sprains? Any personal issues that, will and can, add distractions? Getting back into the swing of things requires gradual, yet deliberate motivation to concentrate on what works and coming to grips with what didn't. Sometimes one pitch may be completely tossed out of a pitcher's repertoire for whatever reason(s). Other times the target that has been used in the past, just isn't producing the kind of results that one expects.
All of what I mentioned here can be overkill for someone in the youth leagues. However, what I've mentioned - and then some, is in orbit for those that take this position seriously and understand the if-ee survival rate of being a pitcher.