By continuing to play Qualifiers and Opens you can earn lots of different rewards:
Additional Invites - Canada is special and has two Tour championships each season, attending one uses a single invite. If you qualify twice, you could theoretically play both Tour championships in a season!
Additional Promos - Every time you place in a qualifying spot you earn a Foil Top Finisher promo. This promo is awarded to all qualifiers, even if you are already qualified.
Additional prizes - All Qualifiers and Opens give some store credit or other prizes for top finishers. Placing well still gives you these rewards.