The knowledge of who is winning can be the enemy. One way of keeping it interesting is to keep secret who is winning. Small World does this and I think it's great. You've seen the guy across the table get a lot of points recently but you aren't sure if he is winning.
Another thing a lot of games do along the same lines is have bonus victory points. You can see Player A is clearly in the lead but player B might have enough bonus victory points to pass her. Many games involve something like this. Railroad Tycoon and Revolution are two that I have and can think of right now. In Railroad Tycoon the players have a secret mission to complete to score some extra victory points. In Revolution, it's a little more open. If a player wants, he can count and see who is going to win after the bonuses are added. I think most people just look at the track and guess who is winning from that and what spots they control.
"Well, I'm done. Bill is clearly gonna win." This can make it not fun to continue playing for some players. However, most people keep playing and start playing for second place. They eliminate that player in the lead and start a "new" game with everyone below him. This makes the game interesting again. Hopefully, the group of people you play with fall into this category.
A catch up mechanic can be used to prevent a runaway leader. It helps prevent that player way in the back saying it's pointless and quitting or trying to ruin the game. That player in last always needs to have at least a small chance of winning. If they can't win it becomes boring. The same can be said for the person in first. They might be so far ahead there becomes no challenge. All players need a chance of victory and defeat no matter how small of a chance those are.
Video games with racing institute the most catch up mechanics. If you're way out in front, those cars behind suddenly have ace drivers behind the wheel. The programmers want to make sure you have some fear of losing. It's kinda frustrating when that guy you should be lapping is now on your tail though, but that fear of losing is meant to keep you interested.
The fine line though is to make sure your catchup mechanic doesn't make those last 99 turns useless and it comes down to turn 100. That "next point wins" or "next score is worth more "addition at the end of an outdoor game when people want to end it. It's not exactly fair to the people in the lead and the entire first part of the game was meaningless. Most of the time you play those games for fun and it doesn't really matter. It would be interesting to watch a football game where if you score in the last 2 minutes of the game it's worth 3 touchdowns instead of 1.
The whole point of the mechanic is to make sure there is still fun and interest in the game. Sometimes they aren't tuned properly but the creator had the right idea. Keep people involved and having fun. So, if you are adding one, make sure it works and if not, make sure the people losing they are still having fun.
Play games. Always.