You have to pay an entry fee for entering a race. Winning will give you this entry fee back - and you also win your Rival’s entry fee.
Winning a race will also give you Flags - these are used to unlock higher series (see series and flags FAQ). Every time you win a race, you will receive a crate as well.
When a Clubs exhibition is running, you will also gain Reputation - the higher the Series, the more Reputation will be earned. If you win in your highest unlocked Series, you will gain three Trophies too, used for progressing through Pit Pass. One extra Trophy is awarded for having the fastest lap time, regardless of the race’s outcome.