From Huffington Post, who asks the same question:
“Thus she throws a populist bone to the Warrenite Dems on the left. But since her campaign is already seeking contributions from everyday Americans like me, I can’t understand why our champion will need $2.5 billion of our money to gain the White House. This figure, which is what each major party reportedly plans to spend in quest of the presidency, is more than twice the record amount spent to re-elect President Obama in 2012, and about $700 million more than the total spent for both Romney and Obama in that election. Why does Hillary need $2.5 billion to run for president?”
http://www.huffingtonpost.com/james-heffernan/why-does-hillary-need-25-_b_7056586.html
“Why does Hillary need $2.5 billion to run for president?”
So she can funnel off a huge chunk for herself and Bill. Also some to pay for her defense when she is sued by whomever gets the balls to do so. Some for hush money.