Top 5 Reasons to Powerlevel:
1) Ninja Exam rewards Magas and better scrolls. You can increase your powerlevel using these things. You need to wait at least 20 days to get enough Kage scrolls, plus longer for the +1, +2, +3, and +4. The sooner you start farming the better because it will mean more power as you reset Ninja Exams every day and get the same rewards all over again.
2) Higher level players get more money. Kyuubi rewards hundreds of thousands of experience and gold, trying your best to achieve high ranks means you have the cash to afford more upgrades as opposed to forcibly staying low leveled to farm more gold.
3) Tendo/Sage treasures only unlock at level 50 and 70. If you want free pull attempts to get Pains or Sage Naruto then you're going to want to level up. The sooner you hit 70, the sooner you can start gambling. Since you're guaranteed to get rare ninja after X number of pulls anyway because the chance keeps going up after each failure then you want to start as early as possible.
4) The Great Ninja War rewards players who WIN. You can get up to 10x Seal Scrolls for a legendary pack, along with like 5000 group tokens. Those tokens can be used to purchase and upgrade ninja like Kisame while the Seal scrolls give you a chance at pulling them for free! For any free player, winning great ninja war is like receiving $500 to divide among your guild. Low levels don't win Ninja War.
5) Accessing higher elite instances gives you access to better farmable ninja. Sure, stay low level if you want to use Team 7 for the next two years. But if you want access to Orochimaru, Jiraiya, and Tsunade then you'll need to be at least level 60. Since they take a MONTH of farming to get, the sooner you unlock the instances the sooner you can start.