Parliamentary opposition in modern political systems plays a large role, particularly in the field of legislation and control of the work of the government or ministry.
Well, the main function of the official opposition party in British style democracy is to OPPOSE. Over the long run, this can have some strange consequences. As governments shift ground to protect their majority over time, the opposition tends to slide into the areas vacated. Also, if an opposition has some original ideas that prove popular, the government may well seek to claim those ideas as their own.
Over time the party of the left becomes the party of the right (from old labour to new labour) and the party of the right may well shift to the left (the Conservatives now talk the language of caring and the like not heard from them since before Thatcher. It's a strange world, if you live long enough!