Well consider this, you have just promoted up to div 1 and all the teams there have 10 times better players than you. Now you can either try to win a few games through the season and upgrade your team constantly and probably still relegate as the teams are just that much better, or you can prepare for relegation, cut down on salaries so you increase income, train efficiently, receive a great draftee, relegate and dominate in the lower league and then come back up with a superior team to hopefully allow you to stay in div 1.
I'm not saying you should always tank if you are the worst team in the league, I'm just saying that it can be a viable option.