Position sizing has two purposes: preserve your capital, and avoid major losses. The most common form of position sizing is using a fixed percentage. This is important, as chaos in trading is the same as throwing money in the air and hoping you have more when it comes down. The right position sizing is one of the factors between making money or going bust.
While a sizeable payday is the dream of any trader, there is the obvious risk of if that trade goes sideways. If it all falls apart, without consideration given to the size of the position they put into the trade concerning their overall portfolio, there could be a significant problem. Applying strict rules, prevents a loss from disrupting your whole plan.