You will learn something about a topic that interests you. You could meet people with similar interests. You might win one of the many great prizes and trips. Lots of ordinary kids win prizes.
Hard work is needed.
Parents and teachers can help. Projects can be quite simple, eg. The Cornwall section of the Chemical Institute of Canada started the local Science Fair in 1976. There are 12 dedicated volunteers who run the science fair. They meet monthly from Sept. to June.