You chances of making it in the money in a tournament is generally somewhere between 10-20% depending on your skills and the level of players in the tourney. Now the chances of winning a tournament are very slim, but the majority of your yearly results can consist of top places you get in large field tournaments. For this reasons, even expert MTT players can experience swings of tens, even hundreds of buy-ins in tournament poker. You can literally go on playing thousand tournaments and never finish as the winner. Thus, it's easy to see the benefits of following a conservative bankroll management system.
In short, it's better to prepare for experiencing some extreme swings in tourneys, especially those with large fields. Therefore you should have a good bankroll management system to play in sustainable way - you can read more about that in our bankroll management article.