Anime OPs are generally made with the express purpose of setting up expectations for the viewer on what the show is about. Often you may see scenes from the show itself being spliced into the OP as previews for the ensuing awesomeness that lies in wait. Some OPs, however, can be so striking and memorable on their own that they alone invite you to return and re-watch the series again and again.
No matter how many times I listen to ‘Sorairo Days’ from Tengen Toppa Gurren Lagann the awesomeness reigns true. My heart grows to the size of the Super Galaxy Gurren Lagann every time I hear the opening line “Kimi ha kikoeru?”.