It isn't just about getting near the top.
Having properly implemented leaderboards for every track means you can go to time trial, do laps with any car you want at any track you want, and compare your time with others that have a similar lap time, see which cars they use, whether they use stock cars or upgraded cars, which driving aids they use, and so forth. You could also set the leaderboards to show only players that use cars within a certain class, or stock cars only, or restrict to only players who used the same car you're using. You can download ghosts of other players, watch their replays and see where they're faster than you (one can learn a lot more watching ghosts of good players than doing silly license tests with cars you'll never use). You can see which upgrades they use, which setup they use, and hopefully learn for them.
If PD did something like Forza's rivals mode you could do all of the above, have fun competing against your friends & strangers even when they're not online, and earn credits for it.
If PD included leaderboards for career mode like Shift 2 did you could compare yourself to your friends, see which cars they use for each career event, and earn bonus credits for beating their best career results.
All of that makes the game more interesting and increases replay value.