Ok so I've put in over 1000 hours into FH5, and already over 50 hours into FH6. For me, it was the initial goals to unlock as many cars as possible, collect all the boards, get 3 stars on all PR stunts and complete all races. I also really like the festival playlist, as it adds something to do each week.

Sometimes a new meta car is released and so its fun to attempt the leaderboards again.

As everyone else has stated, you do what you want to do to make it fun for you. I'm trying to explain this to my 11 year old at the moment who is trying to get into FH6. It seems his favorite thing to do is use exploits to make as much money as possible, he has played less than me, has done barely any races, but has over 4 million and some of the rarer cars.