A Cedar Fair Platinum Pass gets you into all the Cedar Fair parks (not just the one you purchased the pass at). It not only covers the park entrance, but also includes free parking (some parks like Kings Dominion & Kings Island even have a separate Platinum Pass parking area that's close to the park entrance) & ERT (at Cedar Point, Kings Island & Kings Dominion).
If you are going to more than one Cedar Fair park this year then you should consider getting one. Well worth it IMO.