You want to make course adjustments months or years in advance, which may involve things like gravity assists and slingshots that require very good predictions of where multiple different bodies will be a year from now. It can be thousands of times more expensive to course-correct "just in time", or even not possible at all.
As with most discussions of orbital mechanics, the best advice I can give if you want to learn more is: play Kerbal Space Program (1, not 2)
As with most discussions of orbital mechanics, the best advice I can give if you want to learn more is: play Kerbal Space Program (1, not 2)