The main reason people back campaigns is because they want to see what is being proposed happen.
Other than that though, there are a number of things you will receive for backing a campaign.
Everyone who backs a campaign receives advanced access to witness the video proof of a campaign, before anyone who didn't support the campaign can.
You will also receive trophies along the campaign journey, from the moment you back it to the moment it ends. These are based on different events throughout the campaign journey as well as how much you may spend on a specific campaign.
There are also options to earn Leaderboard perks, which are created and set by the campaign creators. Some of these might include being invited to witness a Cajole, getting a shoutout by the target when they complete their Cajole, or whatever the creator can imagine.