It's all about tax breaks. This is a blatant attempt by Amazon to get years of taxes abated in order to provide the increased regional economic benefit of... doing exactly what they'd do anyways if they didn't get the break. US city and state then competes against other US cities and states in a battle to provide the larger tax break.
It's an odious practice in my opinion, and I wish lawmakers would do something to curb it. But being able to tell your constituents "Hey, look at me, I got Amazon to come here!" makes for great campaigning.