As someone who's taken (and continues to take) various stimulants to manage ADD: Because you build up a tolerance over time, which requires larger doses, and b

As someone who's taken (and continues to take) various stimulants to manage ADD: Because you build up a tolerance over time, which requires larger doses, and because untreated ADD/ADHD is also linked to increased creativity and some degree of impulsiveness (which can be a good thing, vs. indecisiveness). If I were doing more creative work, I would probably go without during creative phases and with while producing (for example, as a journalist, go without to come up with ideas for stories, then back on to burn through the associated research, production, refining, etc.). At higher doses, I'm able to fixate on one task to the exclusion of all else (e.g. distractions, music, hunger, illness); at moderate doses, I tend to flit between interesting topics until I find something I can easily focus on (or perhaps until the medication kicks in at full strength).