If you like more than a few political pages on Facebook, you might have noticed a trend. "Sign the petition to stop Hillary Clinton's lies! Sign the petition to stop Republicans from defunding Planned Parenthood." The political left, right, and center want you to sign their petitions, but these aren't your typical petitions.
These aren't actual petitions where you need to sign your name to get a measure on the ballot or repeal a law. These petitions are actually totally ineffective at doing anything except giving away your data.
In fact, these petitions are made specifically to collect data. At a minimum they require you to enter your email address in order to "sign", but more often than not they want your physical address too. Political groups use this information to identify their strongest supporters and build up their email lists. From NPR:
"Nothing comes close," to an email list, said Michael Beach, co-founder of Targeted Victory, a digital campaign firm that works with Republicans. Beach worked with Mitt Romney's campaign in 2012 and now works with Rick Perry's campaign. "Our campaigns will do 70 percent plus of their fundraising through email," he said.
On the Democratic side, the Obama camp took 90 percent of their online money from emails in 2012.
That's why you're being asked to sign all these online petitions. If a campaign or PAC can get your email, they have a cheap and effective way to ask you for money.